mirror of https://github.com/python/cpython.git
2799 lines
100 KiB
TeX
2799 lines
100 KiB
TeX
\documentstyle[twoside,11pt,myformat]{report}
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\title{\bf Python Reference Manual}
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\author{
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Guido van Rossum \\
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Dept. CST, CWI, Kruislaan 413 \\
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1098 SJ Amsterdam, The Netherlands \\
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E-mail: {\tt guido@cwi.nl}
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}
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% Tell \index to actually write the .idx file
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\makeindex
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\begin{document}
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\pagenumbering{roman}
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\maketitle
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\begin{abstract}
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\noindent
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Python is a simple, yet powerful, interpreted programming language
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that bridges the gap between C and shell programming, and is thus
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ideally suited for ``throw-away programming'' and rapid prototyping.
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Its syntax is put together from constructs borrowed from a variety of
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other languages; most prominent are influences from ABC, C, Modula-3
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and Icon.
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The Python interpreter is easily extended with new functions and data
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types implemented in C. Python is also suitable as an extension
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language for highly customizable C applications such as editors or
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window managers.
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Python is available for various operating systems, amongst which
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several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,
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and MS-DOS.
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This reference manual describes the syntax and ``core semantics'' of
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the language. It is terse, but attempts to be exact and complete.
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The semantics of non-essential built-in object types and of the
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built-in functions and modules are described in the {\em Python
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Library Reference}. For an informal introduction to the language, see
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the {\em Python Tutorial}.
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\end{abstract}
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\pagebreak
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{
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\parskip = 0mm
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\tableofcontents
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}
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\pagebreak
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\pagenumbering{arabic}
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\chapter{Introduction}
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This reference manual describes the Python programming language.
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It is not intended as a tutorial.
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While I am trying to be as precise as possible, I chose to use English
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rather than formal specifications for everything except syntax and
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lexical analysis. This should make the document better understandable
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to the average reader, but will leave room for ambiguities.
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Consequently, if you were coming from Mars and tried to re-implement
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Python from this document alone, you might have to guess things and in
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fact you would probably end up implementing quite a different language.
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On the other hand, if you are using
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Python and wonder what the precise rules about a particular area of
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the language are, you should definitely be able to find them here.
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It is dangerous to add too many implementation details to a language
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reference document --- the implementation may change, and other
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implementations of the same language may work differently. On the
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other hand, there is currently only one Python implementation, and
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its particular quirks are sometimes worth being mentioned, especially
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where the implementation imposes additional limitations. Therefore,
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you'll find short ``implementation notes'' sprinkled throughout the
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text.
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Every Python implementation comes with a number of built-in and
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standard modules. These are not documented here, but in the separate
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{\em Python Library Reference} document. A few built-in modules are
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mentioned when they interact in a significant way with the language
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definition.
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\section{Notation}
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The descriptions of lexical analysis and syntax use a modified BNF
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grammar notation. This uses the following style of definition:
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\index{BNF}
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\index{grammar}
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\index{syntax}
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\index{notation}
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\begin{verbatim}
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name: lc_letter (lc_letter | "_")*
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lc_letter: "a"..."z"
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\end{verbatim}
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The first line says that a \verb\name\ is an \verb\lc_letter\ followed by
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a sequence of zero or more \verb\lc_letter\s and underscores. An
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\verb\lc_letter\ in turn is any of the single characters `a' through `z'.
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(This rule is actually adhered to for the names defined in lexical and
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grammar rules in this document.)
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Each rule begins with a name (which is the name defined by the rule)
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and a colon. A vertical bar
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(\verb\|\) is used to separate alternatives; it is the least binding
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operator in this notation. A star (\verb\*\) means zero or more
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repetitions of the preceding item; likewise, a plus (\verb\+\) means
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one or more repetitions, and a question mark (\verb\?\) zero or one
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(in other words, the preceding item is optional). These three
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operators bind as tightly as possible; parentheses are used for
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grouping. Literal strings are enclosed in double quotes. White space
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is only meaningful to separate tokens. Rules are normally contained
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on a single line; rules with many alternatives may be formatted
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alternatively with each line after the first beginning with a
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vertical bar.
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In lexical definitions (as the example above), two more conventions
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are used: Two literal characters separated by three dots mean a choice
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of any single character in the given (inclusive) range of ASCII
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characters. A phrase between angular brackets (\verb\<...>\) gives an
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informal description of the symbol defined; e.g. this could be used
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to describe the notion of `control character' if needed.
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\index{lexical definitions}
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\index{ASCII}
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Even though the notation used is almost the same, there is a big
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difference between the meaning of lexical and syntactic definitions:
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a lexical definition operates on the individual characters of the
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input source, while a syntax definition operates on the stream of
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tokens generated by the lexical analysis. All uses of BNF in the next
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chapter (``Lexical Analysis'') are lexical definitions; uses in
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subsequenc chapter are syntactic definitions.
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\chapter{Lexical analysis}
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A Python program is read by a {\em parser}. Input to the parser is a
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stream of {\em tokens}, generated by the {\em lexical analyzer}. This
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chapter describes how the lexical analyzer breaks a file into tokens.
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\index{lexical analysis}
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\index{parser}
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\index{token}
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\section{Line structure}
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A Python program is divided in a number of logical lines. The end of
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a logical line is represented by the token NEWLINE. Statements cannot
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cross logical line boundaries except where NEWLINE is allowed by the
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syntax (e.g. between statements in compound statements).
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\index{line structure}
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\index{logical line}
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\index{NEWLINE token}
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\subsection{Comments}
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A comment starts with a hash character (\verb\#\) that is not part of
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a string literal, and ends at the end of the physical line. A comment
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always signifies the end of the logical line. Comments are ignored by
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the syntax.
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\index{comment}
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\index{logical line}
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\index{physical line}
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\index{hash character}
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\subsection{Line joining}
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Two or more physical lines may be joined into logical lines using
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backslash characters (\verb/\/), as follows: when a physical line ends
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in a backslash that is not part of a string literal or comment, it is
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joined with the following forming a single logical line, deleting the
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backslash and the following end-of-line character. For example:
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\index{physical line}
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\index{line joining}
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\index{backslash character}
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%
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\begin{verbatim}
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moth_names = ['Januari', 'Februari', 'Maart', \
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'April', 'Mei', 'Juni', \
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'Juli', 'Augustus', 'September', \
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'Oktober', 'November', 'December']
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\end{verbatim}
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\subsection{Blank lines}
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A logical line that contains only spaces, tabs, and possibly a
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comment, is ignored (i.e., no NEWLINE token is generated), except that
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during interactive input of statements, an entirely blank logical line
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terminates a multi-line statement.
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\index{blank line}
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\subsection{Indentation}
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Leading whitespace (spaces and tabs) at the beginning of a logical
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line is used to compute the indentation level of the line, which in
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turn is used to determine the grouping of statements.
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\index{indentation}
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\index{whitespace}
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\index{leading whitespace}
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\index{space}
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\index{tab}
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\index{grouping}
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\index{statement grouping}
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First, tabs are replaced (from left to right) by one to eight spaces
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such that the total number of characters up to there is a multiple of
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eight (this is intended to be the same rule as used by {\UNIX}). The
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total number of spaces preceding the first non-blank character then
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determines the line's indentation. Indentation cannot be split over
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multiple physical lines using backslashes.
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The indentation levels of consecutive lines are used to generate
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INDENT and DEDENT tokens, using a stack, as follows.
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\index{INDENT token}
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\index{DEDENT token}
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Before the first line of the file is read, a single zero is pushed on
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the stack; this will never be popped off again. The numbers pushed on
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the stack will always be strictly increasing from bottom to top. At
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the beginning of each logical line, the line's indentation level is
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compared to the top of the stack. If it is equal, nothing happens.
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If it larger, it is pushed on the stack, and one INDENT token is
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generated. If it is smaller, it {\em must} be one of the numbers
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occurring on the stack; all numbers on the stack that are larger are
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popped off, and for each number popped off a DEDENT token is
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generated. At the end of the file, a DEDENT token is generated for
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each number remaining on the stack that is larger than zero.
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Here is an example of a correctly (though confusingly) indented piece
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of Python code:
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\begin{verbatim}
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def perm(l):
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# Compute the list of all permutations of l
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if len(l) <= 1:
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return [l]
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r = []
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for i in range(len(l)):
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s = l[:i] + l[i+1:]
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p = perm(s)
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for x in p:
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r.append(l[i:i+1] + x)
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return r
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\end{verbatim}
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The following example shows various indentation errors:
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\begin{verbatim}
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def perm(l): # error: first line indented
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for i in range(len(l)): # error: not indented
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s = l[:i] + l[i+1:]
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p = perm(l[:i] + l[i+1:]) # error: unexpected indent
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for x in p:
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r.append(l[i:i+1] + x)
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return r # error: inconsistent dedent
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\end{verbatim}
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(Actually, the first three errors are detected by the parser; only the
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last error is found by the lexical analyzer --- the indentation of
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\verb\return r\ does not match a level popped off the stack.)
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\section{Other tokens}
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Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
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exist: identifiers, keywords, literals, operators, and delimiters.
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Spaces and tabs are not tokens, but serve to delimit tokens. Where
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ambiguity exists, a token comprises the longest possible string that
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forms a legal token, when read from left to right.
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\section{Identifiers}
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Identifiers (also referred to as names) are described by the following
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lexical definitions:
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\index{identifier}
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\index{name}
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\begin{verbatim}
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identifier: (letter|"_") (letter|digit|"_")*
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letter: lowercase | uppercase
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lowercase: "a"..."z"
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uppercase: "A"..."Z"
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digit: "0"..."9"
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\end{verbatim}
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Identifiers are unlimited in length. Case is significant.
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\subsection{Keywords}
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The following identifiers are used as reserved words, or {\em
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keywords} of the language, and cannot be used as ordinary
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identifiers. They must be spelled exactly as written here:
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\index{keyword}
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\index{reserved word}
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\begin{verbatim}
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and del for in print
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break elif from is raise
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class else global not return
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continue except if or try
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def finally import pass while
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\end{verbatim}
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% # This Python program sorts and formats the above table
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% import string
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% l = []
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% try:
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% while 1:
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% l = l + string.split(raw_input())
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% except EOFError:
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% pass
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% l.sort()
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% for i in range((len(l)+4)/5):
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% for j in range(i, len(l), 5):
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% print string.ljust(l[j], 10),
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% print
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\section{Literals} \label{literals}
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Literals are notations for constant values of some built-in types.
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\index{literal}
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\index{constant}
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\subsection{String literals}
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String literals are described by the following lexical definitions:
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\index{string literal}
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\begin{verbatim}
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stringliteral: "'" stringitem* "'"
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stringitem: stringchar | escapeseq
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stringchar: <any ASCII character except newline or "\" or "'">
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escapeseq: "'" <any ASCII character except newline>
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\end{verbatim}
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\index{ASCII}
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String literals cannot span physical line boundaries. Escape
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sequences in strings are actually interpreted according to rules
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simular to those used by Standard C. The recognized escape sequences
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are:
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\index{physical line}
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\index{escape sequence}
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\index{Standard C}
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\index{C}
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\begin{center}
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\begin{tabular}{|l|l|}
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\hline
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\verb/\\/ & Backslash (\verb/\/) \\
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\verb/\'/ & Single quote (\verb/'/) \\
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\verb/\a/ & ASCII Bell (BEL) \\
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\verb/\b/ & ASCII Backspace (BS) \\
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%\verb/\E/ & ASCII Escape (ESC) \\
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\verb/\f/ & ASCII Formfeed (FF) \\
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\verb/\n/ & ASCII Linefeed (LF) \\
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\verb/\r/ & ASCII Carriage Return (CR) \\
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\verb/\t/ & ASCII Horizontal Tab (TAB) \\
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\verb/\v/ & ASCII Vertical Tab (VT) \\
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\verb/\/{\em ooo} & ASCII character with octal value {\em ooo} \\
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\verb/\x/{\em xx...} & ASCII character with hex value {\em xx...} \\
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\hline
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\end{tabular}
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\end{center}
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\index{ASCII}
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In strict compatibility with in Standard C, up to three octal digits are
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accepted, but an unlimited number of hex digits is taken to be part of
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the hex escape (and then the lower 8 bits of the resulting hex number
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are used in all current implementations...).
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All unrecognized escape sequences are left in the string unchanged,
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i.e., {\em the backslash is left in the string.} (This behavior is
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useful when debugging: if an escape sequence is mistyped, the
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resulting output is more easily recognized as broken. It also helps a
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great deal for string literals used as regular expressions or
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otherwise passed to other modules that do their own escape handling.)
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\index{unrecognized escape sequence}
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\subsection{Numeric literals}
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There are three types of numeric literals: plain integers, long
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integers, and floating point numbers.
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\index{number}
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\index{numeric literal}
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\index{integer literal}
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\index{plain integer literal}
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\index{long integer literal}
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\index{floating point literal}
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\index{hexadecimal literal}
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\index{octal literal}
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\index{decimal literal}
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Integer and long integer literals are described by the following
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lexical definitions:
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\begin{verbatim}
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longinteger: integer ("l"|"L")
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integer: decimalinteger | octinteger | hexinteger
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decimalinteger: nonzerodigit digit* | "0"
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octinteger: "0" octdigit+
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hexinteger: "0" ("x"|"X") hexdigit+
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nonzerodigit: "1"..."9"
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octdigit: "0"..."7"
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hexdigit: digit|"a"..."f"|"A"..."F"
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\end{verbatim}
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Although both lower case `l' and upper case `L' are allowed as suffix
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for long integers, it is strongly recommended to always use `L', since
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the letter `l' looks too much like the digit `1'.
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Plain integer decimal literals must be at most $2^{31} - 1$ (i.e., the
|
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largest positive integer, assuming 32-bit arithmetic). Plain octal and
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hexadecimal literals may be as large as $2^{32} - 1$, but values
|
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larger than $2^{31} - 1$ are converted to a negative value by
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subtracting $2^{32}$. There is no limit for long integer literals.
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Some examples of plain and long integer literals:
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\begin{verbatim}
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7 2147483647 0177 0x80000000
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3L 79228162514264337593543950336L 0377L 0x100000000L
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\end{verbatim}
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|
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Floating point literals are described by the following lexical
|
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definitions:
|
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|
\begin{verbatim}
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floatnumber: pointfloat | exponentfloat
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pointfloat: [intpart] fraction | intpart "."
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exponentfloat: (intpart | pointfloat) exponent
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intpart: digit+
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fraction: "." digit+
|
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exponent: ("e"|"E") ["+"|"-"] digit+
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\end{verbatim}
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|
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The allowed range of floating point literals is
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implementation-dependent.
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|
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Some examples of floating point literals:
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|
|
\begin{verbatim}
|
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3.14 10. .001 1e100 3.14e-10
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\end{verbatim}
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|
|
|
Note that numeric literals do not include a sign; a phrase like
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\verb\-1\ is actually an expression composed of the operator
|
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\verb\-\ and the literal \verb\1\.
|
|
|
|
\section{Operators}
|
|
|
|
The following tokens are operators:
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\index{operators}
|
|
|
|
\begin{verbatim}
|
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+ - * / %
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<< >> & | ^ ~
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< == > <= <> != >=
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\end{verbatim}
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|
|
The comparison operators \verb\<>\ and \verb\!=\ are alternate
|
|
spellings of the same operator.
|
|
|
|
\section{Delimiters}
|
|
|
|
The following tokens serve as delimiters or otherwise have a special
|
|
meaning:
|
|
\index{delimiters}
|
|
|
|
\begin{verbatim}
|
|
( ) [ ] { }
|
|
; , : . ` =
|
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\end{verbatim}
|
|
|
|
The following printing ASCII characters are not used in Python. Their
|
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occurrence outside string literals and comments is an unconditional
|
|
error:
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\index{ASCII}
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|
|
\begin{verbatim}
|
|
! @ $ " ?
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\end{verbatim}
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|
|
They may be used by future versions of the language though!
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|
|
\chapter{Data model}
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|
|
|
\section{Objects, values and types}
|
|
|
|
{\em Objects} are Python's abstraction for data. All data in a Python
|
|
program is represented by objects or by relations between objects.
|
|
(In a sense, and in conformance to Von Neumann's model of a
|
|
``stored program computer'', code is also represented by objects.)
|
|
\index{object}
|
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\index{data}
|
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|
|
Every object has an identity, a type and a value. An object's {\em
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|
identity} never changes once it has been created; you may think of it
|
|
as the object's address in memory. An object's {\em type} is also
|
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unchangeable. It determines the operations that an object supports
|
|
(e.g. ``does it have a length?'') and also defines the possible
|
|
values for objects of that type. The {\em value} of some objects can
|
|
change. Objects whose value can change are said to be {\em mutable};
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|
objects whose value is unchangeable once they are created are called
|
|
{\em immutable}. The type determines an object's (im)mutability.
|
|
\index{identity of an object}
|
|
\index{value of an object}
|
|
\index{type of an object}
|
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\index{mutable object}
|
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\index{immutable object}
|
|
|
|
Objects are never explicitly destroyed; however, when they become
|
|
unreachable they may be garbage-collected. An implementation is
|
|
allowed to delay garbage collection or omit it altogether --- it is a
|
|
matter of implementation quality how garbage collection is
|
|
implemented, as long as no objects are collected that are still
|
|
reachable. (Implementation note: the current implementation uses a
|
|
reference-counting scheme which collects most objects as soon as they
|
|
become unreachable, but never collects garbage containing circular
|
|
references.)
|
|
\index{garbage collection}
|
|
\index{reference counting}
|
|
\index{unreachable object}
|
|
|
|
Note that the use of the implementation's tracing or debugging
|
|
facilities may keep objects alive that would normally be collectable.
|
|
|
|
Some objects contain references to ``external'' resources such as open
|
|
files or windows. It is understood that these resources are freed
|
|
when the object is garbage-collected, but since garbage collection is
|
|
not guaranteed to happen, such objects also provide an explicit way to
|
|
release the external resource, usually a \verb\close\ method.
|
|
Programs are strongly recommended to always explicitly close such
|
|
objects.
|
|
|
|
Some objects contain references to other objects; these are called
|
|
{\em containers}. Examples of containers are tuples, lists and
|
|
dictionaries. The references are part of a container's value. In
|
|
most cases, when we talk about the value of a container, we imply the
|
|
values, not the identities of the contained objects; however, when we
|
|
talk about the (im)mutability of a container, only the identities of
|
|
the immediately contained objects are implied. (So, if an immutable
|
|
container contains a reference to a mutable object, its value changes
|
|
if that mutable object is changed.)
|
|
\index{container}
|
|
|
|
Types affect almost all aspects of objects' lives. Even the meaning
|
|
of object identity is affected in some sense: for immutable types,
|
|
operations that compute new values may actually return a reference to
|
|
any existing object with the same type and value, while for mutable
|
|
objects this is not allowed. E.g. after
|
|
|
|
\begin{verbatim}
|
|
a = 1; b = 1; c = []; d = []
|
|
\end{verbatim}
|
|
|
|
\verb\a\ and \verb\b\ may or may not refer to the same object with the
|
|
value one, depending on the implementation, but \verb\c\ and \verb\d\
|
|
are guaranteed to refer to two different, unique, newly created empty
|
|
lists.
|
|
|
|
\section{The standard type hierarchy} \label{types}
|
|
|
|
Below is a list of the types that are built into Python. Extension
|
|
modules written in C can define additional types. Future versions of
|
|
Python may add types to the type hierarchy (e.g. rational or complex
|
|
numbers, efficiently stored arrays of integers, etc.).
|
|
\index{type}
|
|
\indexii{data}{type}
|
|
\indexii{type}{hierarchy}
|
|
\indexii{extension}{module}
|
|
\index{C}
|
|
|
|
Some of the type descriptions below contain a paragraph listing
|
|
`special attributes'. These are attributes that provide access to the
|
|
implementation and are not intended for general use. Their definition
|
|
may change in the future. There are also some `generic' special
|
|
attributes, not listed with the individual objects: \verb\__methods__\
|
|
is a list of the method names of a built-in object, if it has any;
|
|
\verb\__members__\ is a list of the data attribute names of a built-in
|
|
object, if it has any.
|
|
\index{attribute}
|
|
\indexii{special}{attribute}
|
|
\indexiii{generic}{special}{attribute}
|
|
\ttindex{__methods__}
|
|
\ttindex{__members__}
|
|
|
|
\begin{description}
|
|
|
|
\item[None]
|
|
This type has a single value. There is a single object with this value.
|
|
This object is accessed through the built-in name \verb\None\.
|
|
It is returned from functions that don't explicitly return an object.
|
|
\ttindex{None}
|
|
\obindex{None@{\tt None}}
|
|
|
|
\item[Numbers]
|
|
These are created by numeric literals and returned as results by
|
|
arithmetic operators and arithmetic built-in functions. Numeric
|
|
objects are immutable; once created their value never changes. Python
|
|
numbers are of course strongly related to mathematical numbers, but
|
|
subject to the limitations of numerical representation in computers.
|
|
\obindex{number}
|
|
\obindex{numeric}
|
|
|
|
Python distinguishes between integers and floating point numbers:
|
|
|
|
\begin{description}
|
|
\item[Integers]
|
|
These represent elements from the mathematical set of whole numbers.
|
|
\obindex{integer}
|
|
|
|
There are two types of integers:
|
|
|
|
\begin{description}
|
|
|
|
\item[Plain integers]
|
|
These represent numbers in the range $-2^{31}$ through $2^{31}-1$.
|
|
(The range may be larger on machines with a larger natural word
|
|
size, but not smaller.)
|
|
When the result of an operation falls outside this range, the
|
|
exception \verb\OverflowError\ is raised.
|
|
For the purpose of shift and mask operations, integers are assumed to
|
|
have a binary, 2's complement notation using 32 or more bits, and
|
|
hiding no bits from the user (i.e., all $2^{32}$ different bit
|
|
patterns correspond to different values).
|
|
\obindex{plain integer}
|
|
|
|
\item[Long integers]
|
|
These represent numbers in an unlimited range, subject to avaiable
|
|
(virtual) memory only. For the purpose of shift and mask operations,
|
|
a binary representation is assumed, and negative numbers are
|
|
represented in a variant of 2's complement which gives the illusion of
|
|
an infinite string of sign bits extending to the left.
|
|
\obindex{long integer}
|
|
|
|
\end{description} % Integers
|
|
|
|
The rules for integer representation are intended to give the most
|
|
meaningful interpretation of shift and mask operations involving
|
|
negative integers and the least surprises when switching between the
|
|
plain and long integer domains. For any operation except left shift,
|
|
if it yields a result in the plain integer domain without causing
|
|
overflow, it will yield the same result in the long integer domain or
|
|
when using mixed operands.
|
|
\indexii{integer}{representation}
|
|
|
|
\item[Floating point numbers]
|
|
These represent machine-level double precision floating point numbers.
|
|
You are at the mercy of the underlying machine architecture and
|
|
C implementation for the accepted range and handling of overflow.
|
|
\obindex{floating point}
|
|
\indexii{floating point}{number}
|
|
\index{C}
|
|
|
|
\end{description} % Numbers
|
|
|
|
\item[Sequences]
|
|
These represent finite ordered sets indexed by natural numbers.
|
|
The built-in function \verb\len()\ returns the number of elements
|
|
of a sequence. When this number is $n$, the index set contains
|
|
the numbers $0, 1, \ldots, n-1$. Element \verb\i\ of sequence
|
|
\verb\a\ is selected by \verb\a[i]\.
|
|
\obindex{seqence}
|
|
\bifuncindex{len}
|
|
\index{index operation}
|
|
\index{item selection}
|
|
\index{subscription}
|
|
|
|
Sequences also support slicing: \verb\a[i:j]\ selects all elements
|
|
with index $k$ such that $i <= k < j$. When used as an expression,
|
|
a slice is a sequence of the same type --- this implies that the
|
|
index set is renumbered so that it starts at 0 again.
|
|
\index{slicing}
|
|
|
|
Sequences are distinguished according to their mutability:
|
|
|
|
\begin{description}
|
|
%
|
|
\item[Immutable sequences]
|
|
An object of an immutable sequence type cannot change once it is
|
|
created. (If the object contains references to other objects,
|
|
these other objects may be mutable and may be changed; however
|
|
the collection of objects directly referenced by an immutable object
|
|
cannot change.)
|
|
\obindex{immutable sequence}
|
|
\obindex{immutable}
|
|
|
|
The following types are immutable sequences:
|
|
|
|
\begin{description}
|
|
|
|
\item[Strings]
|
|
The elements of a string are characters. There is no separate
|
|
character type; a character is represented by a string of one element.
|
|
Characters represent (at least) 8-bit bytes. The built-in
|
|
functions \verb\chr()\ and \verb\ord()\ convert between characters
|
|
and nonnegative integers representing the byte values.
|
|
Bytes with the values 0-127 represent the corresponding ASCII values.
|
|
The string data type is also used to represent arrays of bytes, e.g.
|
|
to hold data read from a file.
|
|
\obindex{string}
|
|
\index{character}
|
|
\index{byte}
|
|
\index{ASCII}
|
|
\bifuncindex{chr}
|
|
\bifuncindex{ord}
|
|
|
|
(On systems whose native character set is not ASCII, strings may use
|
|
EBCDIC in their internal representation, provided the functions
|
|
\verb\chr()\ and \verb\ord()\ implement a mapping between ASCII and
|
|
EBCDIC, and string comparison preserves the ASCII order.
|
|
Or perhaps someone can propose a better rule?)
|
|
\index{ASCII}
|
|
\index{EBCDIC}
|
|
\index{character set}
|
|
\indexii{string}{comparison}
|
|
\bifuncindex{chr}
|
|
\bifuncindex{ord}
|
|
|
|
\item[Tuples]
|
|
The elements of a tuple are arbitrary Python objects.
|
|
Tuples of two or more elements are formed by comma-separated lists
|
|
of expressions. A tuple of one element (a `singleton') can be formed
|
|
by affixing a comma to an expression (an expression by itself does
|
|
not create a tuple, since parentheses must be usable for grouping of
|
|
expressions). An empty tuple can be formed by enclosing `nothing' in
|
|
parentheses.
|
|
\obindex{tuple}
|
|
\indexii{singleton}{tuple}
|
|
\indexii{empty}{tuple}
|
|
|
|
\end{description} % Immutable sequences
|
|
|
|
\item[Mutable sequences]
|
|
Mutable sequences can be changed after they are created. The
|
|
subscription and slicing notations can be used as the target of
|
|
assignment and \verb\del\ (delete) statements.
|
|
\obindex{mutable sequece}
|
|
\obindex{mutable}
|
|
\indexii{assignment}{statement}
|
|
\index{delete}
|
|
\stindex{del}
|
|
\index{subscription}
|
|
\index{slicing}
|
|
|
|
There is currently a single mutable sequence type:
|
|
|
|
\begin{description}
|
|
|
|
\item[Lists]
|
|
The elements of a list are arbitrary Python objects. Lists are formed
|
|
by placing a comma-separated list of expressions in square brackets.
|
|
(Note that there are no special cases needed to form lists of length 0
|
|
or 1.)
|
|
\obindex{list}
|
|
|
|
\end{description} % Mutable sequences
|
|
|
|
\end{description} % Sequences
|
|
|
|
\item[Mapping types]
|
|
These represent finite sets of objects indexed by arbitrary index sets.
|
|
The subscript notation \verb\a[k]\ selects the element indexed
|
|
by \verb\k\ from the mapping \verb\a\; this can be used in
|
|
expressions and as the target of assignments or \verb\del\ statements.
|
|
The built-in function \verb\len()\ returns the number of elements
|
|
in a mapping.
|
|
\bifuncindex{len}
|
|
\index{subscription}
|
|
\obindex{mapping}
|
|
|
|
There is currently a single mapping type:
|
|
|
|
\begin{description}
|
|
|
|
\item[Dictionaries]
|
|
These represent finite sets of objects indexed by strings.
|
|
Dictionaries are mutable; they are created by the \verb\{...}\
|
|
notation (see section \ref{dict}). (Implementation note: the strings
|
|
used for indexing must not contain null bytes.)
|
|
\obindex{dictionary}
|
|
\obindex{mutable}
|
|
|
|
\end{description} % Mapping types
|
|
|
|
\item[Callable types]
|
|
These are the types to which the function call (invocation) operation,
|
|
written as \verb\function(argument, argument, ...)\, can be applied:
|
|
\indexii{function}{call}
|
|
\index{invocation}
|
|
\indexii{function}{argument}
|
|
\obindex{callable}
|
|
|
|
\begin{description}
|
|
|
|
\item[User-defined functions]
|
|
A user-defined function object is created by a function definition
|
|
(see section \ref{function}). It should be called with an argument
|
|
list containing the same number of items as the function's formal
|
|
parameter list.
|
|
\indexii{user-defined}{function}
|
|
\obindex{function}
|
|
\obindex{user-defined function}
|
|
|
|
Special read-only attributes: \verb\func_code\ is the code object
|
|
representing the compiled function body, and \verb\func_globals\ is (a
|
|
reference to) the dictionary that holds the function's global
|
|
variables --- it implements the global name space of the module in
|
|
which the function was defined.
|
|
\ttindex{func_code}
|
|
\ttindex{func_globals}
|
|
\indexii{global}{name space}
|
|
|
|
\item[User-defined methods]
|
|
A user-defined method (a.k.a. {\em object closure}) is a pair of a
|
|
class instance object and a user-defined function. It should be
|
|
called with an argument list containing one item less than the number
|
|
of items in the function's formal parameter list. When called, the
|
|
class instance becomes the first argument, and the call arguments are
|
|
shifted one to the right.
|
|
\obindex{method}
|
|
\obindex{user-defined method}
|
|
indexii{user-defined}{method}
|
|
\index{object closure}
|
|
|
|
Special read-only attributes: \verb\im_self\ is the class instance
|
|
object, \verb\im_func\ is the function object.
|
|
\ttindex{im_func}
|
|
\ttindex{im_self}
|
|
|
|
\item[Built-in functions]
|
|
A built-in function object is a wrapper around a C function. Examples
|
|
of built-in functions are \verb\len\ and \verb\math.sin\. There
|
|
are no special attributes. The number and type of the arguments are
|
|
determined by the C function.
|
|
\obindex{built-in function}
|
|
\obindex{function}
|
|
\index{C}
|
|
|
|
\item[Built-in methods]
|
|
This is really a different disguise of a built-in function, this time
|
|
containing an object passed to the C function as an implicit extra
|
|
argument. An example of a built-in method is \verb\list.append\ if
|
|
\verb\list\ is a list object.
|
|
\obindex{built-in method}
|
|
\obindex{method}
|
|
\indexii{built-in}{method}
|
|
|
|
\item[Classes]
|
|
Class objects are described below. When a class object is called as a
|
|
parameterless function, a new class instance (also described below) is
|
|
created and returned. The class's initialization function is not
|
|
called --- this is the responsibility of the caller. It is illegal to
|
|
call a class object with one or more arguments.
|
|
\obindex{class}
|
|
\obindex{class instance}
|
|
\obindex{instance}
|
|
\indexii{class object}{call}
|
|
|
|
\end{description}
|
|
|
|
\item[Modules]
|
|
Modules are imported by the \verb\import\ statement (see section
|
|
\ref{import}). A module object is a container for a module's name
|
|
space, which is a dictionary (the same dictionary as referenced by the
|
|
\verb\func_globals\ attribute of functions defined in the module).
|
|
Module attribute references are translated to lookups in this
|
|
dictionary. A module object does not contain the code object used to
|
|
initialize the module (since it isn't needed once the initialization
|
|
is done).
|
|
\stindex{import}
|
|
\obindex{module}
|
|
|
|
Attribute assignment update the module's name space dictionary.
|
|
|
|
Special read-only attributes: \verb\__dict__\ yields the module's name
|
|
space as a dictionary object; \verb\__name__\ yields the module's name
|
|
as a string object.
|
|
\ttindex{__dict__}
|
|
\ttindex{__name__}
|
|
\indexii{module}{name space}
|
|
|
|
\item[Classes]
|
|
Class objects are created by class definitions (see section
|
|
\ref{class}). A class is a container for a dictionary containing the
|
|
class's name space. Class attribute references are translated to
|
|
lookups in this dictionary. When an attribute name is not found
|
|
there, the attribute search continues in the base classes. The search
|
|
is depth-first, left-to-right in the order of their occurrence in the
|
|
base class list.
|
|
\obindex{class}
|
|
\obindex{class instance}
|
|
\obindex{instance}
|
|
\indexii{class object}{call}
|
|
\index{container}
|
|
\index{dictionary}
|
|
\indexii{class}{attribute}
|
|
|
|
Class attribute assignments update the class's dictionary, never the
|
|
dictionary of a base class.
|
|
\indexiii{class}{attribute}{assignment}
|
|
|
|
A class can be called as a parameterless function to yield a class
|
|
instance (see above).
|
|
\indexii{class object}{call}
|
|
|
|
Special read-only attributes: \verb\__dict__\ yields te dictionary
|
|
containing the class's name space; \verb\__bases__\ yields a tuple
|
|
(possibly empty or a singleton) containing the base classes, in the
|
|
order of their occurrence in the base class list.
|
|
\ttindex{__dict__}
|
|
\ttindex{__bases__}
|
|
|
|
\item[Class instances]
|
|
A class instance is created by calling a class object as a
|
|
parameterless function. A class instance has a dictionary in which
|
|
attribute references are searched. When an attribute is not found
|
|
there, and the instance's class has an attribute by that name, and
|
|
that class attribute is a user-defined function (and in no other
|
|
cases), the instance attribute reference yields a user-defined method
|
|
object (see above) constructed from the instance and the function.
|
|
\obindex{class instance}
|
|
\obindex{instance}
|
|
\indexii{class}{instance}
|
|
\indexii{class instance}{attribute}
|
|
|
|
Attribute assignments update the instance's dictionary.
|
|
\indexiii{class instance}{attribute}{assignment}
|
|
|
|
Special read-only attributes: \verb\__dict__\ yields the attribute
|
|
dictionary; \verb\__class__\ yields the instance's class.
|
|
\ttindex{__dict__}
|
|
\ttindex{__class__}
|
|
|
|
\item[Files]
|
|
A file object represents an open file. (It is a wrapper around a C
|
|
{\tt stdio} file pointer.) File objects are created by the
|
|
\verb\open()\ built-in function, and also by \verb\posix.popen()\ and
|
|
the \verb\makefile\ method of socket objects. \verb\sys.stdin\,
|
|
\verb\sys.stdout\ and \verb\sys.stderr\ are file objects corresponding
|
|
the the interpreter's standard input, output and error streams.
|
|
See the Python Library Reference for methods of file objects and other
|
|
details.
|
|
\obindex{file}
|
|
\index{C}
|
|
\index{stdio}
|
|
\bifuncindex{open}
|
|
\bifuncindex{popen}
|
|
\bifuncindex{makefile}
|
|
\ttindex{stdin}
|
|
\ttindex{stdout}
|
|
\ttindex{stderr}
|
|
\ttindex{sys.stdin}
|
|
\ttindex{sys.stdout}
|
|
\ttindex{sys.stderr}
|
|
|
|
\item[Internal types]
|
|
A few types used internally by the interpreter are exposed to the user.
|
|
Their definition may change with future versions of the interpreter,
|
|
but they are mentioned here for completeness.
|
|
\index{internal type}
|
|
|
|
\begin{description}
|
|
|
|
\item[Code objects]
|
|
Code objects represent executable code. The difference between a code
|
|
object and a function object is that the function object contains an
|
|
explicit reference to the function's context (the module in which it
|
|
was defined) which a code object contains no context. There is no way
|
|
to execute a bare code object.
|
|
\obindex{code}
|
|
|
|
Special read-only attributes: \verb\co_code\ is a string representing
|
|
the sequence of instructions; \verb\co_consts\ is a list of literals
|
|
used by the code; \verb\co_names\ is a list of names (strings) used by
|
|
the code; \verb\co_filename\ is the filename from which the code was
|
|
compiled. (To find out the line numbers, you would have to decode the
|
|
instructions; the standard library module \verb\dis\ contains an
|
|
example of how to do this.)
|
|
\ttindex{co_code}
|
|
\ttindex{co_consts}
|
|
\ttindex{co_names}
|
|
\ttindex{co_filename}
|
|
|
|
\item[Frame objects]
|
|
Frame objects represent execution frames. They may occur in traceback
|
|
objects (see below).
|
|
\obindex{frame}
|
|
|
|
Special read-only attributes: \verb\f_back\ is to the previous
|
|
stack frame (towards the caller), or \verb\None\ if this is the bottom
|
|
stack frame; \verb\f_code\ is the code object being executed in this
|
|
frame; \verb\f_globals\ is the dictionary used to look up global
|
|
variables; \verb\f_locals\ is used for local variables;
|
|
\verb\f_lineno\ gives the line number and \verb\f_lasti\ gives the
|
|
precise instruction (this is an index into the instruction string of
|
|
the code object).
|
|
\ttindex{f_back}
|
|
\ttindex{f_code}
|
|
\ttindex{f_globals}
|
|
\ttindex{f_locals}
|
|
\ttindex{f_lineno}
|
|
\ttindex{f_lasti}
|
|
|
|
\item[Traceback objects]
|
|
Traceback objects represent a stack trace of an exception. A
|
|
traceback object is created when an exception occurs. When the search
|
|
for an exception handler unwinds the execution stack, at each unwound
|
|
level a traceback object is inserted in front of the current
|
|
traceback. When an exception handler is entered, the stack trace is
|
|
made available to the program as \verb\sys.exc_traceback\. When the
|
|
program contains no suitable handler, the stack trace is written
|
|
(nicely formatted) to the standard error stream; if the interpreter is
|
|
interactive, it is also made available to the user as
|
|
\verb\sys.last_traceback\.
|
|
\obindex{traceback}
|
|
\indexii{stack}{trace}
|
|
\indexii{exception}{handler}
|
|
\indexii{execution}{stack}
|
|
\ttindex{exc_traceback}
|
|
\ttindex{last_traceback}
|
|
\ttindex{sys.exc_traceback}
|
|
\ttindex{sys.last_traceback}
|
|
|
|
Special read-only attributes: \verb\tb_next\ is the next level in the
|
|
stack trace (towards the frame where the exception occurred), or
|
|
\verb\None\ if there is no next level; \verb\tb_frame\ points to the
|
|
execution frame of the current level; \verb\tb_lineno\ gives the line
|
|
number where the exception occurred; \verb\tb_lasti\ indicates the
|
|
precise instruction. The line number and last instruction in the
|
|
traceback may differ from the line number of its frame object if the
|
|
exception occurred in a \verb\try\ statement with no matching
|
|
\verb\except\ clause or with a \verb\finally\ clause.
|
|
\ttindex{tb_next}
|
|
\ttindex{tb_frame}
|
|
\ttindex{tb_lineno}
|
|
\ttindex{tb_lasti}
|
|
\stindex{try}
|
|
|
|
\end{description} % Internal types
|
|
|
|
\end{description} % Types
|
|
|
|
\chapter{Execution model}
|
|
\index{execution model}
|
|
|
|
\section{Code blocks, execution frames, and name spaces} \label{execframes}
|
|
\index{code block}
|
|
\indexii{execution}{frame}
|
|
\index{name space}
|
|
|
|
A {\em code block} is a piece of Python program text that can be
|
|
executed as a unit, such as a module, a class definition or a function
|
|
body. Some code blocks (like modules) are executed only once, others
|
|
(like function bodies) may be executed many times. Code block may
|
|
textually contain other code blocks. Code blocks may invoke other
|
|
code blocks (that may or may not be textually contained in them) as
|
|
part of their execution, e.g. by invoking (calling) a function.
|
|
\index{code block}
|
|
\indexii{code}{block}
|
|
|
|
The following are code blocks: A module is a code block. A function
|
|
body is a code block. A class definition is a code block. Each
|
|
command typed interactively is a separate code block; a script file is
|
|
a code block. The string argument passed to the built-in functions
|
|
\verb\eval\ and \verb\exec\ are code blocks. And finally, the
|
|
expression read and evaluated by the built-in function \verb\input\ is
|
|
a code block.
|
|
|
|
A code block is executed in an execution frame. An {\em execution
|
|
frame} contains some administrative information (used for debugging),
|
|
determines where and how execution continues after the code block's
|
|
execution has completed, and (perhaps most importantly) defines two
|
|
name spaces, the local and the global name space, that affect
|
|
execution of the code block.
|
|
\indexii{execution}{frame}
|
|
|
|
A {\em name space} is a mapping from names (identifiers) to objects.
|
|
A particular name space may be referenced by more than one execution
|
|
frame, and from other places as well. Adding a name to a name space
|
|
is called {\em binding} a name (to an object); changing the mapping of
|
|
a name is called {\em rebinding}; removing a name is {\em unbinding}.
|
|
Name spaces are functionally equivalent to dictionaries.
|
|
\index{name space}
|
|
\indexii{binding}{name}
|
|
\indexii{rebinding}{name}
|
|
\indexii{unbinding}{name}
|
|
|
|
The {\em local name space} of an execution frame determines the default
|
|
place where names are defined and searched. The {\em global name
|
|
space} determines the place where names listed in \verb\global\
|
|
statements are defined and searched, and where names that are not
|
|
explicitly bound in the current code block are searched.
|
|
\indexii{local}{name space}
|
|
\indexii{global}{name space}
|
|
\stindex{global}
|
|
|
|
Whether a name is local or global in a code block is determined by
|
|
static inspection of the source text for the code block: in the
|
|
absence of \verb\global\ statements, a name that is bound anywhere in
|
|
the code block is local in the entire code block; all other names are
|
|
considered global. The \verb\global\ statement forces global
|
|
interpretation of selected names throughout the code block. The
|
|
following constructs bind names: formal parameters, \verb\import\
|
|
statements, class and function definitions (these bind the class or
|
|
function name), and targets that are identifiers if occurring in an
|
|
assignment, \verb\for\ loop header, or \verb\except\ clause header.
|
|
(A target occurring in a \verb\del\ statement does not bind a name.)
|
|
|
|
When a global name is not found in the global name space, it is
|
|
searched in the list of ``built-in'' names (which is actually the
|
|
global name space of the module \verb\builtin\). When a name is not
|
|
found at all, the \verb\NameError\ exception is raised.
|
|
|
|
The following table lists the meaning of the local and global name
|
|
space for various types of code blocks. The name space for a
|
|
particular module is automatically created when the module is first
|
|
referenced.
|
|
|
|
\begin{center}
|
|
\begin{tabular}{|l|l|l|l|}
|
|
\hline
|
|
Code block type & Global name space & Local name space & Notes \\
|
|
\hline
|
|
Module & n.s. for this module & same as global & \\
|
|
Script & n.s. for \verb\__main__\ & same as global & \\
|
|
Interactive command & n.s. for \verb\__main__\ & same as global & \\
|
|
Class definition & global n.s. of containing block & new n.s. & \\
|
|
Function body & global n.s. of containing block & new n.s. & \\
|
|
String passed to \verb\exec\ or \verb\eval\
|
|
& global n.s. of caller & local n.s. of caller & (1) \\
|
|
File read by \verb\execfile\
|
|
& global n.s. of caller & local n.s. of caller & (1) \\
|
|
Expression read by \verb\input\
|
|
& global n.s. of caller & local n.s. of caller & \\
|
|
\hline
|
|
\end{tabular}
|
|
\end{center}
|
|
|
|
Notes:
|
|
|
|
\begin{description}
|
|
|
|
\item[n.s.] means {\em name space}
|
|
|
|
\item[(1)] The global and local name space for these functions can be
|
|
overridden with optional extra arguments.
|
|
|
|
\end{description}
|
|
|
|
\section{Exceptions}
|
|
|
|
Exceptions are a means of breaking out of the normal flow of control
|
|
of a code block in order to handle errors or other exceptional
|
|
conditions. An exception is {\em raised} at the point where the error
|
|
is detected; it may be {\em handled} by the surrounding code block or
|
|
by any code block that directly or indirectly invoked the code block
|
|
where the error occurred.
|
|
\index{exception}
|
|
\index{raise an exception}
|
|
\index{handle an exception}
|
|
\index{exception handler}
|
|
\index{errors}
|
|
\index{error handling}
|
|
|
|
The Python interpreter raises an exception when it detects an run-time
|
|
error (such as division by zero). A Python program can also
|
|
explicitly raise an exception with the \verb\raise\ statement.
|
|
Exception handlers are specified with the \verb\try...except\
|
|
statement.
|
|
|
|
Python uses the ``termination'' model of error handling: an exception
|
|
handler can find out what happened and continue execution at an outer
|
|
level, but it cannot repair the cause of the error and retry the
|
|
failing operation (except by re-entering the the offending piece of
|
|
code from the top).
|
|
|
|
When an exception is not handled at all, the interpreter terminates
|
|
execution of the program, or returns to its interactive main loop.
|
|
|
|
Exceptions are identified by string objects. Two different string
|
|
objects with the same value identify different exceptions.
|
|
|
|
When an exception is raised, an object (maybe \verb\None\) is passed
|
|
as the exception's ``parameter''; this object does not affect the
|
|
selection of an exception handler, but is passed to the selected
|
|
exception handler as additional information.
|
|
|
|
See also the description of the \verb\try\ and \verb\raise\
|
|
statements.
|
|
|
|
\chapter{Expressions and conditions}
|
|
\index{expression}
|
|
\index{condition}
|
|
|
|
{\bf Note:} In this and the following chapters, extended BNF notation
|
|
will be used to describe syntax, not lexical analysis.
|
|
\index{BNF}
|
|
|
|
This chapter explains the meaning of the elements of expressions and
|
|
conditions. Conditions are a superset of expressions, and a condition
|
|
may be used wherever an expression is required by enclosing it in
|
|
parentheses. The only places where expressions are used in the syntax
|
|
instead of conditions is in expression statements and on the
|
|
right-hand side of assignment statements; this catches some nasty bugs
|
|
like accedentally writing \verb\x == 1\ instead of \verb\x = 1\.
|
|
\indexii{assignment}{statement}
|
|
|
|
The comma plays several roles in Python's syntax. It is usually an
|
|
operator with a lower precedence than all others, but occasionally
|
|
serves other purposes as well; e.g. it separates function arguments,
|
|
is used in list and dictionary constructors, and has special semantics
|
|
in \verb\print\ statements.
|
|
\index{comma}
|
|
|
|
When (one alternative of) a syntax rule has the form
|
|
|
|
\begin{verbatim}
|
|
name: othername
|
|
\end{verbatim}
|
|
|
|
and no semantics are given, the semantics of this form of \verb\name\
|
|
are the same as for \verb\othername\.
|
|
\index{syntax}
|
|
|
|
\section{Arithmetic conversions}
|
|
\indexii{arithmetic}{conversion}
|
|
|
|
When a description of an arithmetic operator below uses the phrase
|
|
``the numeric arguments are converted to a common type'',
|
|
this both means that if either argument is not a number, a
|
|
\verb\TypeError\ exception is raised, and that otherwise
|
|
the following conversions are applied:
|
|
\exindex{TypeError}
|
|
\indexii{floating point}{number}
|
|
\indexii{long}{integer}
|
|
\indexii{plain}{integer}
|
|
|
|
\begin{itemize}
|
|
\item first, if either argument is a floating point number,
|
|
the other is converted to floating point;
|
|
\item else, if either argument is a long integer,
|
|
the other is converted to long integer;
|
|
\item otherwise, both must be plain integers and no conversion
|
|
is necessary.
|
|
\end{itemize}
|
|
|
|
\section{Atoms}
|
|
\index{atom}
|
|
|
|
Atoms are the most basic elements of expressions. Forms enclosed in
|
|
reverse quotes or in parentheses, brackets or braces are also
|
|
categorized syntactically as atoms. The syntax for atoms is:
|
|
|
|
\begin{verbatim}
|
|
atom: identifier | literal | enclosure
|
|
enclosure: parenth_form | list_display | dict_display | string_conversion
|
|
\end{verbatim}
|
|
|
|
\subsection{Identifiers (Names)}
|
|
\index{name}
|
|
\index{identifier}
|
|
|
|
An identifier occurring as an atom is a reference to a local, global
|
|
or built-in name binding. If a name can be assigned to anywhere in a
|
|
code block, and is not mentioned in a \verb\global\ statement in that
|
|
code block, it refers to a local name throughout that code block.
|
|
Otherwise, it refers to a global name if one exists, else to a
|
|
built-in name.
|
|
\indexii{name}{binding}
|
|
\index{code block}
|
|
\stindex{global}
|
|
\indexii{built-in}{name}
|
|
\indexii{global}{name}
|
|
|
|
When the name is bound to an object, evaluation of the atom yields
|
|
that object. When a name is not bound, an attempt to evaluate it
|
|
raises a \verb\NameError\ exception.
|
|
\exindex{NameError}
|
|
|
|
\subsection{Literals}
|
|
\index{literal}
|
|
|
|
Python knows string and numeric literals:
|
|
|
|
\begin{verbatim}
|
|
literal: stringliteral | integer | longinteger | floatnumber
|
|
\end{verbatim}
|
|
|
|
Evaluation of a literal yields an object of the given type (string,
|
|
integer, long integer, floating point number) with the given value.
|
|
The value may be approximated in the case of floating point literals.
|
|
See section \ref{literals} for details.
|
|
|
|
All literals correspond to immutable data types, and hence the
|
|
object's identity is less important than its value. Multiple
|
|
evaluations of literals with the same value (either the same
|
|
occurrence in the program text or a different occurrence) may obtain
|
|
the same object or a different object with the same value.
|
|
\indexiii{immutable}{data}{type}
|
|
|
|
(In the original implementation, all literals in the same code block
|
|
with the same type and value yield the same object.)
|
|
|
|
\subsection{Parenthesized forms}
|
|
\index{parenthesized form}
|
|
|
|
A parenthesized form is an optional condition list enclosed in
|
|
parentheses:
|
|
|
|
\begin{verbatim}
|
|
parenth_form: "(" [condition_list] ")"
|
|
\end{verbatim}
|
|
|
|
A parenthesized condition list yields whatever that condition list
|
|
yields.
|
|
|
|
An empty pair of parentheses yields an empty tuple object. Since
|
|
tuples are immutable, the rules for literals apply here.
|
|
\indexii{empty}{tuple}
|
|
|
|
(Note that tuples are not formed by the parentheses, but rather by use
|
|
of the comma operator. The exception is the empty tuple, for which
|
|
parentheses {\em are} required --- allowing unparenthesized ``nothing''
|
|
in expressions would causes ambiguities and allow common typos to
|
|
pass uncaught.)
|
|
\index{comma}
|
|
\index{tuple}{display}
|
|
|
|
\subsection{List displays}
|
|
\indexii{list}{display}
|
|
|
|
A list display is a possibly empty series of conditions enclosed in
|
|
square brackets:
|
|
|
|
\begin{verbatim}
|
|
list_display: "[" [condition_list] "]"
|
|
\end{verbatim}
|
|
|
|
A list display yields a new list object.
|
|
\obindex{list}
|
|
|
|
If it has no condition list, the list object has no items. Otherwise,
|
|
the elements of the condition list are evaluated from left to right
|
|
and inserted in the list object in that order.
|
|
\indexii{empty}{list}
|
|
|
|
\subsection{Dictionary displays} \label{dict}
|
|
\indexii{dictionary}{display}
|
|
|
|
A dictionary display is a possibly empty series of key/datum pairs
|
|
enclosed in curly braces:
|
|
\index{key}
|
|
\index{datum}
|
|
\index{key/datum pair}
|
|
|
|
\begin{verbatim}
|
|
dict_display: "{" [key_datum_list] "}"
|
|
key_datum_list: [key_datum ("," key_datum)* [","]
|
|
key_datum: condition ":" condition
|
|
\end{verbatim}
|
|
|
|
A dictionary display yields a new dictionary object.
|
|
\obindex{dictionary}
|
|
|
|
The key/datum pairs are evaluated from left to right to define the
|
|
entries of the dictionary: each key object is used as a key into the
|
|
dictionary to store the corresponding datum.
|
|
|
|
Keys must be strings, otherwise a \verb\TypeError\ exception is
|
|
raised. Clashes between duplicate keys are not detected; the last
|
|
datum (textually rightmost in the display) stored for a given key
|
|
value prevails.
|
|
\exindex{TypeError}
|
|
|
|
\subsection{String conversions}
|
|
\indexii{string}{conversion}
|
|
|
|
A string conversion is a condition list enclosed in reverse (or
|
|
backward) quotes:
|
|
|
|
\begin{verbatim}
|
|
string_conversion: "`" condition_list "`"
|
|
\end{verbatim}
|
|
|
|
A string conversion evaluates the contained condition list and
|
|
converts the resulting object into a string according to rules
|
|
specific to its type.
|
|
|
|
If the object is a string, a number, \verb\None\, or a tuple, list or
|
|
dictionary containing only objects whose type is one of these, the
|
|
resulting string is a valid Python expression which can be passed to
|
|
the built-in function \verb\eval()\ to yield an expression with the
|
|
same value (or an approximation, if floating point numbers are
|
|
involved).
|
|
|
|
(In particular, converting a string adds quotes around it and converts
|
|
``funny'' characters to escape sequences that are safe to print.)
|
|
|
|
It is illegal to attempt to convert recursive objects (e.g. lists or
|
|
dictionaries that contain a reference to themselves, directly or
|
|
indirectly.)
|
|
\obindex{recursive}
|
|
|
|
\section{Primaries} \label{primaries}
|
|
\index{primary}
|
|
|
|
Primaries represent the most tightly bound operations of the language.
|
|
Their syntax is:
|
|
|
|
\begin{verbatim}
|
|
primary: atom | attributeref | subscription | slicing | call
|
|
\end{verbatim}
|
|
|
|
\subsection{Attribute references}
|
|
\indexii{attribute}{reference}
|
|
|
|
An attribute reference is a primary followed by a period and a name:
|
|
|
|
\begin{verbatim}
|
|
attributeref: primary "." identifier
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to an object of a type that supports
|
|
attribute references, e.g. a module or a list. This object is then
|
|
asked to produce the attribute whose name is the identifier. If this
|
|
attribute is not available, the exception \verb\AttributeError\ is
|
|
raised. Otherwise, the type and value of the object produced is
|
|
determined by the object. Multiple evaluations of the same attribute
|
|
reference may yield different objects.
|
|
\obindex{module}
|
|
\obindex{list}
|
|
|
|
\subsection{Subscriptions}
|
|
\index{subscription}
|
|
|
|
A subscription selects an item of a sequence (string, tuple or list)
|
|
or mapping (dictionary) object:
|
|
\obindex{sequence}
|
|
\obindex{mapping}
|
|
\obindex{string}
|
|
\obindex{tuple}
|
|
\obindex{list}
|
|
\obindex{dictionary}
|
|
\indexii{sequence}{item}
|
|
|
|
\begin{verbatim}
|
|
subscription: primary "[" condition "]"
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to an object of a sequence or mapping type.
|
|
|
|
If it is a mapping, the condition must evaluate to an object whose
|
|
value is one of the keys of the mapping, and the subscription selects
|
|
the value in the mapping that corresponds to that key.
|
|
|
|
If it is a sequence, the condition must evaluate to a plain integer.
|
|
If this value is negative, the length of the sequence is added to it
|
|
(so that, e.g. \verb\x[-1]\ selects the last item of \verb\x\.)
|
|
The resulting value must be a nonnegative integer smaller than the
|
|
number of items in the sequence, and the subscription selects the item
|
|
whose index is that value (counting from zero).
|
|
|
|
A string's items are characters. A character is not a separate data
|
|
type but a string of exactly one character.
|
|
\index{character}
|
|
\indexii{string}{item}
|
|
|
|
\subsection{Slicings}
|
|
\index{slicing}
|
|
\index{slice}
|
|
|
|
A slicing (or slice) selects a range of items in a sequence (string,
|
|
tuple or list) object:
|
|
\obindex{sequence}
|
|
\obindex{string}
|
|
\obindex{tuple}
|
|
\obindex{list}
|
|
|
|
\begin{verbatim}
|
|
slicing: primary "[" [condition] ":" [condition] "]"
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to a sequence object. The lower and upper
|
|
bound expressions, if present, must evaluate to plain integers;
|
|
defaults are zero and the sequence's length, respectively. If either
|
|
bound is negative, the sequence's length is added to it. The slicing
|
|
now selects all items with index $k$ such that $i <= k < j$ where $i$
|
|
and $j$ are the specified lower and upper bounds. This may be an
|
|
empty sequence. It is not an error if $i$ or $j$ lie outside the
|
|
range of valid indexes (such items don't exist so they aren't
|
|
selected).
|
|
|
|
\subsection{Calls} \label{calls}
|
|
\index{call}
|
|
|
|
A call calls a callable object (e.g. a function) with a possibly empty
|
|
series of arguments:
|
|
\obindex{callable}
|
|
|
|
\begin{verbatim}
|
|
call: primary "(" [condition_list] ")"
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to a callable object (user-defined
|
|
functions, built-in functions, methods of built-in objects, class
|
|
objects, and methods of class instances are callable). If it is a
|
|
class, the argument list must be empty; otherwise, the arguments are
|
|
evaluated.
|
|
|
|
A call always returns some value, possibly \verb\None\, unless it
|
|
raises an exception. How this value is computed depends on the type
|
|
of the callable object. If it is:
|
|
|
|
\begin{description}
|
|
|
|
\item[a user-defined function:] the code block for the function is
|
|
executed, passing it the argument list. The first thing the code
|
|
block will do is bind the formal parameters to the arguments; this is
|
|
described in section \ref{function}. When the code block executes a
|
|
\verb\return\ statement, this specifies the return value of the
|
|
function call.
|
|
\indexii{function}{call}
|
|
\indexiii{user-defined}{function}{call}
|
|
\obindex{user-defined function}
|
|
\obindex{function}
|
|
|
|
\item[a built-in function or method:] the result is up to the
|
|
interpreter; see the library reference manual for the descriptions of
|
|
built-in functions and methods.
|
|
\indexii{function}{call}
|
|
\indexii{built-in function}{call}
|
|
\indexii{method}{call}
|
|
\indexii{built-in method}{call}
|
|
\obindex{built-in method}
|
|
\obindex{built-in function}
|
|
\obindex{method}
|
|
\obindex{function}
|
|
|
|
\item[a class object:] a new instance of that class is returned.
|
|
\obindex{class}
|
|
\indexii{class object}{call}
|
|
|
|
\item[a class instance method:] the corresponding user-defined
|
|
function is called, with an argument list that is one longer than the
|
|
argument list of the call: the instance becomes the first argument.
|
|
\obindex{class instance}
|
|
\obindex{instance}
|
|
\indexii{instance}{call}
|
|
\indexii{class instance}{call}
|
|
|
|
\end{description}
|
|
|
|
\section{Unary arithmetic operations}
|
|
\indexiii{unary}{arithmetic}{operation}
|
|
\indexiii{unary}{bit-wise}{operation}
|
|
|
|
All unary arithmetic (and bit-wise) operations have the same priority:
|
|
|
|
\begin{verbatim}
|
|
u_expr: primary | "-" u_expr | "+" u_expr | "~" u_expr
|
|
\end{verbatim}
|
|
|
|
The unary \verb\"-"\ (minus) operator yields the negation of its
|
|
numeric argument.
|
|
\index{negation}
|
|
\index{minus}
|
|
|
|
The unary \verb\"+"\ (plus) operator yields its numeric argument
|
|
unchanged.
|
|
\index{plus}
|
|
|
|
The unary \verb\"~"\ (invert) operator yields the bit-wise inversion
|
|
of its plain or long integer argument. The bit-wise inversion of
|
|
\verb\x\ is defined as \verb\-(x+1)\.
|
|
\index{inversion}
|
|
|
|
In all three cases, if the argument does not have the proper type,
|
|
a \verb\TypeError\ exception is raised.
|
|
\exindex{TypeError}
|
|
|
|
\section{Binary arithmetic operations}
|
|
\indexiii{binary}{arithmetic}{operation}
|
|
|
|
The binary arithmetic operations have the conventional priority
|
|
levels. Note that some of these operations also apply to certain
|
|
non-numeric types. There is no ``power'' operator, so there are only
|
|
two levels, one for multiplicative operators and one for additive
|
|
operators:
|
|
|
|
\begin{verbatim}
|
|
m_expr: u_expr | m_expr "*" u_expr | m_expr "/" u_expr | m_expr "%" u_expr
|
|
a_expr: m_expr | aexpr "+" m_expr | aexpr "-" m_expr
|
|
\end{verbatim}
|
|
|
|
The \verb\"*"\ (multiplication) operator yields the product of its
|
|
arguments. The arguments must either both be numbers, or one argument
|
|
must be a plain integer and the other must be a sequence. In the
|
|
former case, the numbers are converted to a common type and then
|
|
multiplied together. In the latter case, sequence repetition is
|
|
performed; a negative repetition factor yields an empty sequence.
|
|
\index{multiplication}
|
|
|
|
The \verb\"/"\ (division) operator yields the quotient of its
|
|
arguments. The numeric arguments are first converted to a common
|
|
type. Plain or long integer division yields an integer of the same
|
|
type; the result is that of mathematical division with the `floor'
|
|
function applied to the result. Division by zero raises the
|
|
\verb\ZeroDivisionError\ exception.
|
|
\exindex{ZeroDivisionError}
|
|
\index{division}
|
|
|
|
The \verb\"%"\ (modulo) operator yields the remainder from the
|
|
division of the first argument by the second. The numeric arguments
|
|
are first converted to a common type. A zero right argument raises
|
|
the \verb\ZeroDivisionError\ exception. The arguments may be floating
|
|
point numbers, e.g. \verb\3.14 % 0.7\ equals \verb\0.34\. The modulo
|
|
operator always yields a result with the same sign as its second
|
|
operand (or zero); the absolute value of the result is strictly
|
|
smaller than the second operand.
|
|
\index{modulo}
|
|
|
|
The integer division and modulo operators are connected by the
|
|
following identity: \verb\x == (x/y)*y + (x%y)\. Integer division and
|
|
modulo are also connected with the built-in function \verb\divmod()\:
|
|
\verb\divmod(x, y) == (x/y, x%y)\. These identities don't hold for
|
|
floating point numbers; there a similar identity holds where
|
|
\verb\x/y\ is replaced by \verb\floor(x/y)\).
|
|
|
|
The \verb\"+"\ (addition) operator yields the sum of its arguments.
|
|
The arguments must either both be numbers, or both sequences of the
|
|
same type. In the former case, the numbers are converted to a common
|
|
type and then added together. In the latter case, the sequences are
|
|
concatenated.
|
|
\index{addition}
|
|
|
|
The \verb\"-"\ (subtraction) operator yields the difference of its
|
|
arguments. The numeric arguments are first converted to a common
|
|
type.
|
|
\index{subtraction}
|
|
|
|
\section{Shifting operations}
|
|
\indexii{shifting}{operation}
|
|
|
|
The shifting operations have lower priority than the arithmetic
|
|
operations:
|
|
|
|
\begin{verbatim}
|
|
shift_expr: a_expr | shift_expr ( "<<" | ">>" ) a_expr
|
|
\end{verbatim}
|
|
|
|
These operators accept plain or long integers as arguments. The
|
|
arguments are converted to a common type. They shift the first
|
|
argument to the left or right by the number of bits given by the
|
|
second argument.
|
|
|
|
A right shift by $n$ bits is defined as division by $2^n$. A left
|
|
shift by $n$ bits is defined as multiplication with $2^n$; for plain
|
|
integers there is no overflow check so this drops bits and flip the
|
|
sign if the result is not less than $2^{31}$ in absolute value.
|
|
|
|
Negative shift counts raise a \verb\ValueError\ exception.
|
|
\exindex{ValueError}
|
|
|
|
\section{Binary bit-wise operations}
|
|
\indexiii{binary}{bit-wise}{operation}
|
|
|
|
Each of the three bitwise operations has a different priority level:
|
|
|
|
\begin{verbatim}
|
|
and_expr: shift_expr | and_expr "&" shift_expr
|
|
xor_expr: and_expr | xor_expr "^" and_expr
|
|
or_expr: xor_expr | or_expr "|" xor_expr
|
|
\end{verbatim}
|
|
|
|
The \verb\"&"\ operator yields the bitwise AND of its arguments, which
|
|
must be plain or long integers. The arguments are converted to a
|
|
common type.
|
|
\indexii{bit-wise}{and}
|
|
|
|
The \verb\"^"\ operator yields the bitwise XOR (exclusive OR) of its
|
|
arguments, which must be plain or long integers. The arguments are
|
|
converted to a common type.
|
|
\indexii{bit-wise}{xor}
|
|
\indexii{exclusive}{or}
|
|
|
|
The \verb\"|"\ operator yields the bitwise (inclusive) OR of its
|
|
arguments, which must be plain or long integers. The arguments are
|
|
converted to a common type.
|
|
\indexii{bit-wise}{or}
|
|
\indexii{inclusive}{or}
|
|
|
|
\section{Comparisons}
|
|
\index{comparison}
|
|
|
|
Contrary to C, all comparison operations in Python have the same
|
|
priority, which is lower than that of any arithmetic, shifting or
|
|
bitwise operation. Also contrary to C, expressions like
|
|
\verb\a < b < c\ have the interpretation that is conventional in
|
|
mathematics:
|
|
\index{C}
|
|
|
|
\begin{verbatim}
|
|
comparison: or_expr (comp_operator or_expr)*
|
|
comp_operator: "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
|
|
\end{verbatim}
|
|
|
|
Comparisons yield integer values: 1 for true, 0 for false.
|
|
|
|
Comparisons can be chained arbitrarily, e.g. $x < y <= z$ is
|
|
equivalent to $x < y$ \verb\and\ $y <= z$, except that $y$ is
|
|
evaluated only once (but in both cases $z$ is not evaluated at all
|
|
when $x < y$ is found to be false).
|
|
\indexii{chaining}{comparisons}
|
|
|
|
Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to
|
|
$e_0 op_1 e_1$ \verb\and\ $e_1 op_2 e_2$ \verb\and\ ... \verb\and\
|
|
$e_{n-1} op_n e_n$, except that each expression is evaluated at most once.
|
|
|
|
Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison
|
|
between $e_0$ and $e_2$, e.g. $x < y > z$ is perfectly legal.
|
|
|
|
The forms \verb\<>\ and \verb\!=\ are equivalent; for consistency with
|
|
C, \verb\!=\ is preferred; where \verb\!=\ is mentioned below
|
|
\verb\<>\ is also implied.
|
|
|
|
The operators {\tt "<", ">", "==", ">=", "<="}, and {\tt "!="} compare
|
|
the values of two objects. The objects needn't have the same type.
|
|
If both are numbers, they are coverted to a common type. Otherwise,
|
|
objects of different types {\em always} compare unequal, and are
|
|
ordered consistently but arbitrarily.
|
|
|
|
(This unusual definition of comparison is done to simplify the
|
|
definition of operations like sorting and the \verb\in\ and \verb\not
|
|
in\ operators.)
|
|
|
|
Comparison of objects of the same type depends on the type:
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
Numbers are compared arithmetically.
|
|
|
|
\item
|
|
Strings are compared lexicographically using the numeric equivalents
|
|
(the result of the built-in function \verb\ord\) of their characters.
|
|
|
|
\item
|
|
Tuples and lists are compared lexicographically using comparison of
|
|
corresponding items.
|
|
|
|
\item
|
|
Mappings (dictionaries) are compared through lexicographic
|
|
comparison of their sorted (key, value) lists.%
|
|
\footnote{This is expensive since it requires sorting the keys first,
|
|
but about the only sensible definition. It was tried to compare
|
|
dictionaries by identity only, but this caused surprises because
|
|
people expected to be able to test a dictionary for emptiness by
|
|
comparing it to {\tt \{\}}.}
|
|
|
|
\item
|
|
Most other types compare unequal unless they are the same object;
|
|
the choice whether one object is considered smaller or larger than
|
|
another one is made arbitrarily but consistently within one
|
|
execution of a program.
|
|
|
|
\end{itemize}
|
|
|
|
The operators \verb\in\ and \verb\not in\ test for sequence
|
|
membership: if $y$ is a sequence, $x ~\verb\in\~ y$ is true if and
|
|
only if there exists an index $i$ such that $x = y[i]$.
|
|
$x ~\verb\not in\~ y$ yields the inverse truth value. The exception
|
|
\verb\TypeError\ is raised when $y$ is not a sequence, or when $y$ is
|
|
a string and $x$ is not a string of length one.%
|
|
\footnote{The latter restriction is sometimes a nuisance.}
|
|
\opindex{in}
|
|
\opindex{not in}
|
|
\indexii{membership}{test}
|
|
\obindex{sequence}
|
|
|
|
The operators \verb\is\ and \verb\is not\ test for object identity:
|
|
$x ~\verb\is\~ y$ is true if and only if $x$ and $y$ are the same
|
|
object. $x ~\verb\is not\~ y$ yields the inverse truth value.
|
|
\opindex{is}
|
|
\opindex{is not}
|
|
\indexii{identity}{test}
|
|
|
|
\section{Boolean operations} \label{Booleans}
|
|
\indexii{Boolean}{operation}
|
|
|
|
Boolean operations have the lowest priority of all Python operations:
|
|
|
|
\begin{verbatim}
|
|
condition: or_test
|
|
or_test: and_test | or_test "or" and_test
|
|
and_test: not_test | and_test "and" not_test
|
|
not_test: comparison | "not" not_test
|
|
\end{verbatim}
|
|
|
|
In the context of Boolean operations, and also when conditions are
|
|
used by control flow statements, the following values are interpreted
|
|
as false: \verb\None\, numeric zero of all types, empty sequences
|
|
(strings, tuples and lists), and empty mappings (dictionaries). All
|
|
other values are interpreted as true.
|
|
|
|
The operator \verb\not\ yields 1 if its argument is false, 0 otherwise.
|
|
\opindex{not}
|
|
|
|
The condition $x ~\verb\and\~ y$ first evaluates $x$; if $x$ is false,
|
|
its value is returned; otherwise, $y$ is evaluated and the resulting
|
|
value is returned.
|
|
\opindex{and}
|
|
|
|
The condition $x ~\verb\or\~ y$ first evaluates $x$; if $x$ is true,
|
|
its value is returned; otherwise, $y$ is evaluated and the resulting
|
|
value is returned.
|
|
\opindex{or}
|
|
|
|
(Note that \verb\and\ and \verb\or\ do not restrict the value and type
|
|
they return to 0 and 1, but rather return the last evaluated argument.
|
|
This is sometimes useful, e.g. if \verb\s\ is a string that should be
|
|
replaced by a default value if it is empty, the expression
|
|
\verb\s or 'foo'\ yields the desired value. Because \verb\not\ has to
|
|
invent a value anyway, it does not bother to return a value of the
|
|
same type as its argument, so e.g. \verb\not 'foo'\ yields \verb\0\,
|
|
not \verb\''\.)
|
|
|
|
\section{Expression lists and condition lists}
|
|
\indexii{expression}{list}
|
|
\indexii{condition}{list}
|
|
|
|
\begin{verbatim}
|
|
expr_list: or_expr ("," or_expr)* [","]
|
|
cond_list: condition ("," condition)* [","]
|
|
\end{verbatim}
|
|
|
|
The only difference between expression lists and condition lists is
|
|
the lowest priority of operators that can be used in them without
|
|
being enclosed in parentheses; condition lists allow all operators,
|
|
while expression lists don't allow comparisons and Boolean operators
|
|
(they do allow bitwise and shift operators though).
|
|
|
|
Expression lists are used in expression statements and assignments;
|
|
condition lists are used everywhere else where a list of
|
|
comma-separated values is required.
|
|
|
|
An expression (condition) list containing at least one comma yields a
|
|
tuple. The length of the tuple is the number of expressions
|
|
(conditions) in the list. The expressions (conditions) are evaluated
|
|
from left to right. (Conditions lists are used syntactically is a few
|
|
places where no tuple is constructed but a list of values is needed
|
|
nevertheless.)
|
|
\obindex{tuple}
|
|
|
|
The trailing comma is required only to create a single tuple (a.k.a. a
|
|
{\em singleton}); it is optional in all other cases. A single
|
|
expression (condition) without a trailing comma doesn't create a
|
|
tuple, but rather yields the value of that expression (condition).
|
|
\indexii{trailing}{comma}
|
|
|
|
(To create an empty tuple, use an empty pair of parentheses:
|
|
\verb\()\.)
|
|
|
|
\chapter{Simple statements}
|
|
\indexii{simple}{statement}
|
|
|
|
Simple statements are comprised within a single logical line.
|
|
Several simple statements may occur on a single line separated
|
|
by semicolons. The syntax for simple statements is:
|
|
|
|
\begin{verbatim}
|
|
simple_stmt: expression_stmt
|
|
| assignment_stmt
|
|
| pass_stmt
|
|
| del_stmt
|
|
| print_stmt
|
|
| return_stmt
|
|
| raise_stmt
|
|
| break_stmt
|
|
| continue_stmt
|
|
| import_stmt
|
|
| global_stmt
|
|
\end{verbatim}
|
|
|
|
\section{Expression statements}
|
|
\indexii{expression}{statement}
|
|
|
|
Expression statements are used (mostly interactively) to compute and
|
|
write a value, or (usually) to call a procedure (a function that
|
|
returns no meaningful result; in Python, procedures return the value
|
|
\verb\None\):
|
|
|
|
\begin{verbatim}
|
|
expression_stmt: expression_list
|
|
\end{verbatim}
|
|
|
|
An expression statement evaluates the expression list (which may be a
|
|
single expression). If the value is not \verb\None\, it is converted
|
|
to a string using the rules for string conversions (expressions in
|
|
reverse quotes), and the resulting string is written to standard
|
|
output (see section \ref{print}) on a line by itself.
|
|
\indexii{expression}{list}
|
|
\ttindex{None}
|
|
\indexii{string}{conversion}
|
|
\index{output}
|
|
\indexii{standard}{output}
|
|
\indexii{writing}{values}
|
|
|
|
(The exception for \verb\None\ is made so that procedure calls, which
|
|
are syntactically equivalent to expressions, do not cause any output.
|
|
A tuple with only \verb\None\ items is written normally.)
|
|
\indexii{procedure}{call}
|
|
|
|
\section{Assignment statements}
|
|
\indexii{assignment}{statement}
|
|
|
|
Assignment statements are used to (re)bind names to values and to
|
|
modify attributes or items of mutable objects:
|
|
\indexii{binding}{name}
|
|
\indexii{rebinding}{name}
|
|
\obindex{mutable}
|
|
\indexii{attribute}{assignment}
|
|
|
|
\begin{verbatim}
|
|
assignment_stmt: (target_list "=")+ expression_list
|
|
target_list: target ("," target)* [","]
|
|
target: identifier | "(" target_list ")" | "[" target_list "]"
|
|
| attributeref | subscription | slicing
|
|
\end{verbatim}
|
|
|
|
(See section \ref{primaries} for the syntax definitions for the last
|
|
three symbols.)
|
|
|
|
An assignment statement evaluates the expression list (remember that
|
|
this can be a single expression or a comma-separated list, the latter
|
|
yielding a tuple) and assigns the single resulting object to each of
|
|
the target lists, from left to right.
|
|
\indexii{expression}{list}
|
|
|
|
Assignment is defined recursively depending on the form of the target
|
|
(list). When a target is part of a mutable object (an attribute
|
|
reference, subscription or slicing), the mutable object must
|
|
ultimately perform the assignment and decide about its validity, and
|
|
may raise an exception if the assignment is unacceptable. The rules
|
|
observed by various types and the exceptions raised are given with the
|
|
definition of the object types (see section \ref{types}).
|
|
\index{target}
|
|
\indexii{target}{list}
|
|
|
|
Assignment of an object to a target list is recursively defined as
|
|
follows.
|
|
\indexiii{target}{list}{assignment}
|
|
|
|
\begin{itemize}
|
|
\item
|
|
If the target list is a single target: the object is assigned to that
|
|
target.
|
|
|
|
\item
|
|
If the target list is a comma-separated list of targets: the object
|
|
must be a tuple with the same number of items as the list contains
|
|
targets, and the items are assigned, from left to right, to the
|
|
corresponding targets.
|
|
|
|
\end{itemize}
|
|
|
|
Assignment of an object to a single target is recursively defined as
|
|
follows.
|
|
|
|
\begin{itemize} % nested
|
|
|
|
\item
|
|
If the target is an identifier (name):
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
If the name does not occur in a \verb\global\ statement in the current
|
|
code block: the name is bound to the object in the current local name
|
|
space.
|
|
\stindex{global}
|
|
|
|
\item
|
|
Otherwise: the name is bound to the object in the current global name
|
|
space.
|
|
|
|
\end{itemize} % nested
|
|
|
|
The name is rebound if it was already bound.
|
|
|
|
\item
|
|
If the target is a target list enclosed in parentheses: the object is
|
|
assigned to that target list as described above.
|
|
|
|
\item
|
|
If the target is a target list enclosed in square brackets: the object
|
|
must be a list with the same number of items as the target list
|
|
contains targets, and its items are assigned, from left to right, to
|
|
the corresponding targets.
|
|
|
|
\item
|
|
If the target is an attribute reference: The primary expression in the
|
|
reference is evaluated. It should yield an object with assignable
|
|
attributes; if this is not the case, \verb\TypeError\ is raised. That
|
|
object is then asked to assign the assigned object to the given
|
|
attribute; if it cannot perform the assignment, it raises an exception
|
|
(usually but not necessarily \verb\AttributeError\).
|
|
\indexii{attribute}{assignment}
|
|
|
|
\item
|
|
If the target is a subscription: The primary expression in the
|
|
reference is evaluated. It should yield either a mutable sequence
|
|
(list) object or a mapping (dictionary) object. Next, the subscript
|
|
expression is evaluated.
|
|
\indexii{subscription}{assignment}
|
|
\obindex{mutable}
|
|
|
|
If the primary is a mutable sequence object (a list), the subscript
|
|
must yield a plain integer. If it is negative, the sequence's length
|
|
is added to it. The resulting value must be a nonnegative integer
|
|
less than the sequence's length, and the sequence is asked to assign
|
|
the assigned object to its item with that index. If the index is out
|
|
of range, \verb\IndexError\ is raised (assignment to a subscripted
|
|
sequence cannot add new items to a list).
|
|
\obindex{sequence}
|
|
\obindex{list}
|
|
|
|
If the primary is a mapping (dictionary) object, the subscript must
|
|
have a type compatible with the mapping's key type, and the mapping is
|
|
then asked to to create a key/datum pair which maps the subscript to
|
|
the assigned object. This can either replace an existing key/value
|
|
pair with the same key value, or insert a new key/value pair (if no
|
|
key with the same value existed).
|
|
\obindex{mapping}
|
|
\obindex{dictionary}
|
|
|
|
\item
|
|
If the target is a slicing: The primary expression in the reference is
|
|
evaluated. It should yield a mutable sequence (list) object. The
|
|
assigned object should be a sequence object of the same type. Next,
|
|
the lower and upper bound expressions are evaluated, insofar they are
|
|
present; defaults are zero and the sequence's length. The bounds
|
|
should evaluate to (small) integers. If either bound is negative, the
|
|
sequence's length is added to it. The resulting bounds are clipped to
|
|
lie between zero and the sequence's length, inclusive. Finally, the
|
|
sequence object is asked to replace the items indicated by the slice
|
|
with the items of the assigned sequence. This may change the
|
|
sequence's length, if it allows it.
|
|
\indexii{slicing}{assignment}
|
|
|
|
\end{itemize}
|
|
|
|
(In the original implementation, the syntax for targets is taken
|
|
to be the same as for expressions, and invalid syntax is rejected
|
|
during the code generation phase, causing less detailed error
|
|
messages.)
|
|
|
|
\section{The {\tt pass} statement}
|
|
\stindex{pass}
|
|
|
|
\begin{verbatim}
|
|
pass_stmt: "pass"
|
|
\end{verbatim}
|
|
|
|
\verb\pass\ is a null operation --- when it is executed, nothing
|
|
happens. It is useful as a placeholder when a statement is
|
|
required syntactically, but no code needs to be executed, for example:
|
|
\indexii{null}{operation}
|
|
|
|
\begin{verbatim}
|
|
def f(arg): pass # a function that does nothing (yet)
|
|
|
|
class C: pass # an class with no methods (yet)
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt del} statement}
|
|
\stindex{del}
|
|
|
|
\begin{verbatim}
|
|
del_stmt: "del" target_list
|
|
\end{verbatim}
|
|
|
|
Deletion is recursively defined very similar to the way assignment is
|
|
defined. Rather that spelling it out in full details, here are some
|
|
hints.
|
|
\indexii{deletion}{target}
|
|
\indexiii{deletion}{target}{list}
|
|
|
|
Deletion of a target list recursively deletes each target, from left
|
|
to right.
|
|
|
|
Deletion of a name removes the binding of that name (which must exist)
|
|
from the local or global name space, depending on whether the name
|
|
occurs in a \verb\global\ statement in the same code block.
|
|
\stindex{global}
|
|
\indexii{unbinding}{name}
|
|
|
|
Deletion of attribute references, subscriptions and slicings
|
|
is passed to the primary object involved; deletion of a slicing
|
|
is in general equivalent to assignment of an empty slice of the
|
|
right type (but even this is determined by the sliced object).
|
|
\indexii{attribute}{deletion}
|
|
|
|
\section{The {\tt print} statement} \label{print}
|
|
\stindex{print}
|
|
|
|
\begin{verbatim}
|
|
print_stmt: "print" [ condition ("," condition)* [","] ]
|
|
\end{verbatim}
|
|
|
|
\verb\print\ evaluates each condition in turn and writes the resulting
|
|
object to standard output (see below). If an object is not a string,
|
|
it is first converted to a string using the rules for string
|
|
conversions. The (resulting or original) string is then written. A
|
|
space is written before each object is (converted and) written, unless
|
|
the output system believes it is positioned at the beginning of a
|
|
line. This is the case: (1) when no characters have yet been written
|
|
to standard output; or (2) when the last character written to standard
|
|
output is \verb/\n/; or (3) when the last write operation on standard
|
|
output was not a \verb\print\ statement. (In some cases it may be
|
|
functional to write an empty string to standard output for this
|
|
reason.)
|
|
\index{output}
|
|
\indexii{writing}{values}
|
|
|
|
A \verb/"\n"/ character is written at the end, unless the \verb\print\
|
|
statement ends with a comma. This is the only action if the statement
|
|
contains just the keyword \verb\print\.
|
|
\indexii{trailing}{comma}
|
|
\indexii{newline}{suppression}
|
|
|
|
Standard output is defined as the file object named \verb\stdout\
|
|
in the built-in module \verb\sys\. If no such object exists,
|
|
or if it is not a writable file, a \verb\RuntimeError\ exception is raised.
|
|
(The original implementation attempts to write to the system's original
|
|
standard output instead, but this is not safe, and should be fixed.)
|
|
\indexii{standard}{output}
|
|
\bimodindex{sys}
|
|
\ttindex{stdout}
|
|
\exindex{RuntimeError}
|
|
|
|
\section{The {\tt return} statement}
|
|
\stindex{return}
|
|
|
|
\begin{verbatim}
|
|
return_stmt: "return" [condition_list]
|
|
\end{verbatim}
|
|
|
|
\verb\return\ may only occur syntactically nested in a function
|
|
definition, not within a nested class definition.
|
|
\indexii{function}{definition}
|
|
\indexii{class}{definition}
|
|
|
|
If a condition list is present, it is evaluated, else \verb\None\
|
|
is substituted.
|
|
|
|
\verb\return\ leaves the current function call with the condition
|
|
list (or \verb\None\) as return value.
|
|
|
|
When \verb\return\ passes control out of a \verb\try\ statement
|
|
with a \verb\finally\ clause, that finally clause is executed
|
|
before really leaving the function.
|
|
\kwindex{finally}
|
|
|
|
\section{The {\tt raise} statement}
|
|
\stindex{raise}
|
|
|
|
\begin{verbatim}
|
|
raise_stmt: "raise" condition ["," condition]
|
|
\end{verbatim}
|
|
|
|
\verb\raise\ evaluates its first condition, which must yield
|
|
a string object. If there is a second condition, this is evaluated,
|
|
else \verb\None\ is substituted.
|
|
\index{exception}
|
|
\indexii{raising}{exception}
|
|
|
|
It then raises the exception identified by the first object,
|
|
with the second one (or \verb\None\) as its parameter.
|
|
|
|
\section{The {\tt break} statement}
|
|
\stindex{break}
|
|
|
|
\begin{verbatim}
|
|
break_stmt: "break"
|
|
\end{verbatim}
|
|
|
|
\verb\break\ may only occur syntactically nested in a \verb\for\
|
|
or \verb\while\ loop, not nested in a function or class definition.
|
|
\stindex{for}
|
|
\stindex{while}
|
|
\indexii{loop}{statement}
|
|
|
|
It terminates the neares enclosing loop, skipping the optional
|
|
\verb\else\ clause if the loop has one.
|
|
\kwindex{else}
|
|
|
|
If a \verb\for\ loop is terminated by \verb\break\, the loop control
|
|
target keeps its current value.
|
|
\indexii{loop control}{target}
|
|
|
|
When \verb\break\ passes control out of a \verb\try\ statement
|
|
with a \verb\finally\ clause, that finally clause is executed
|
|
before really leaving the loop.
|
|
\kwindex{finally}
|
|
|
|
\section{The {\tt continue} statement}
|
|
\stindex{continue}
|
|
|
|
\begin{verbatim}
|
|
continue_stmt: "continue"
|
|
\end{verbatim}
|
|
|
|
\verb\continue\ may only occur syntactically nested in a \verb\for\ or
|
|
\verb\while\ loop, not nested in a function or class definition, and
|
|
not nested in the \verb\try\ clause of a \verb\try\ statement with a
|
|
\verb\finally\ clause (it may occur nested in a \verb\except\ or
|
|
\verb\finally\ clause of a \verb\try\ statement though).
|
|
\stindex{for}
|
|
\stindex{while}
|
|
\indexii{loop}{statement}
|
|
\kwindex{finally}
|
|
|
|
It continues with the next cycle of the nearest enclosing loop.
|
|
|
|
\section{The {\tt import} statement} \label{import}
|
|
\stindex{import}
|
|
|
|
\begin{verbatim}
|
|
import_stmt: "import" identifier ("," identifier)*
|
|
| "from" identifier "import" identifier ("," identifier)*
|
|
| "from" identifier "import" "*"
|
|
\end{verbatim}
|
|
|
|
Import statements are executed in two steps: (1) find a module, and
|
|
initialize it if necessary; (2) define a name or names in the local
|
|
name space (of the scope where the \verb\import\ statement occurs).
|
|
The first form (without \verb\from\) repeats these steps for each
|
|
identifier in the list, the \verb\from\ form performs them once, with
|
|
the first identifier specifying the module name.
|
|
\indexii{importing}{module}
|
|
\indexii{name}{binding}
|
|
\kwindex{from}
|
|
|
|
The system maintains a table of modules that have been initialized,
|
|
indexed by module name. (The current implementation makes this table
|
|
accessible as \verb\sys.modules\.) When a module name is found in
|
|
this table, step (1) is finished. If not, a search for a module
|
|
definition is started. This first looks for a built-in module
|
|
definition, and if no built-in module if the given name is found, it
|
|
searches a user-specified list of directories for a file whose name is
|
|
the module name with extension \verb\".py"\. (The current
|
|
implementation uses the list of strings \verb\sys.path\ as the search
|
|
path; it is initialized from the shell environment variable
|
|
\verb\$PYTHONPATH\, with an installation-dependent default.)
|
|
\ttindex{modules}
|
|
\ttindex{sys.modules}
|
|
\indexii{module}{name}
|
|
\indexii{built-in}{module}
|
|
\indexii{user-defined}{module}
|
|
\bimodindex{sys}
|
|
\ttindex{path}
|
|
\ttindex{sys.path}
|
|
\indexii{filename}{extension}
|
|
|
|
If a built-in module is found, its built-in initialization code is
|
|
executed and step (1) is finished. If no matching file is found,
|
|
\verb\ImportError\ is raised. If a file is found, it is parsed,
|
|
yielding an executable code block. If a syntax error occurs,
|
|
\verb\SyntaxError\ is raised. Otherwise, an empty module of the given
|
|
name is created and inserted in the module table, and then the code
|
|
block is executed in the context of this module. Exceptions during
|
|
this execution terminate step (1).
|
|
\indexii{module}{initialization}
|
|
\exindex{SyntaxError}
|
|
\exindex{ImportError}
|
|
\index{code block}
|
|
|
|
When step (1) finishes without raising an exception, step (2) can
|
|
begin.
|
|
|
|
The first form of \verb\import\ statement binds the module name in the
|
|
local name space to the module object, and then goes on to import the
|
|
next identifier, if any. The \verb\from\ from does not bind the
|
|
module name: it goes through the list of identifiers, looks each one
|
|
of them up in the module found in step (1), and binds the name in the
|
|
local name space to the object thus found. If a name is not found,
|
|
\verb\ImportError\ is raised. If the list of identifiers is replaced
|
|
by a star (\verb\*\), all names defined in the module are bound,
|
|
except those beginning with an underscore(\verb\_\).
|
|
\indexii{name}{binding}
|
|
\exindex{ImportError}
|
|
|
|
Names bound by import statements may not occur in \verb\global\
|
|
statements in the same scope.
|
|
\stindex{global}
|
|
|
|
The \verb\from\ form with \verb\*\ may only occur in a module scope.
|
|
\kwindex{from}
|
|
\ttindex{from ... import *}
|
|
|
|
(The current implementation does not enforce the latter two
|
|
restrictions, but programs should not abuse this freedom, as future
|
|
implementations may enforce them or silently change the meaning of the
|
|
program.)
|
|
|
|
\section{The {\tt global} statement} \label{global}
|
|
\stindex{global}
|
|
|
|
\begin{verbatim}
|
|
global_stmt: "global" identifier ("," identifier)*
|
|
\end{verbatim}
|
|
|
|
The \verb\global\ statement is a declaration which holds for the
|
|
entire current scope. It means that the listed identifiers are to be
|
|
interpreted as globals. While {\em using} global names is automatic
|
|
if they are not defined in the local scope, {\em assigning} to global
|
|
names would be impossible without \verb\global\.
|
|
\indexiii{global}{name}{binding}
|
|
|
|
Names listed in a \verb\global\ statement must not be used in the same
|
|
scope before that \verb\global\ statement is executed.
|
|
|
|
Name listed in a \verb\global\ statement must not be defined as formal
|
|
parameters or in a \verb\for\ loop control target, \verb\class\
|
|
definition, function definition, or \verb\import\ statement.
|
|
|
|
(The current implementation does not enforce the latter two
|
|
restrictions, but programs should not abuse this freedom, as future
|
|
implementations may enforce them or silently change the meaning of the
|
|
program.)
|
|
|
|
\chapter{Compound statements}
|
|
\indexii{compound}{statement}
|
|
|
|
Compound statements contain (groups of) other statements; they affect
|
|
or control the execution of those other statements in some way. In
|
|
general, compound statements span multiple lines, although in simple
|
|
incarnations a whole compound statement may be contained in one line.
|
|
|
|
The \verb\if\, \verb\while\ and \verb\for\ statements implement
|
|
traditional control flow constructs. \verb\try\ specifies exception
|
|
handlers and/or cleanup code for a group of statements. Function and
|
|
class definitions are also syntactically compound statements.
|
|
|
|
Compound statements consist of one or more `clauses'. A clause
|
|
consists of a header and a `suite'. The clause headers of a
|
|
particular compound statement are all at the same indentation level.
|
|
Each clause header begins with a uniquely identifying keyword and ends
|
|
with a colon. A suite is a group of statements controlled by a
|
|
clause. A suite can be one or more semicolon-separated simple
|
|
statements on the same line as the header, following the header's
|
|
colon, or it can be one or more indented statements on subsequent
|
|
lines. Only the latter form of suite can contain nested compound
|
|
statements; the following is illegal, mostly because it wouldn't be
|
|
clear to which \verb\if\ clause a following \verb\else\ clause would
|
|
belong:
|
|
\index{clause}
|
|
\index{suite}
|
|
|
|
\begin{verbatim}
|
|
if test1: if test2: print x
|
|
\end{verbatim}
|
|
|
|
Also note that the semicolon binds tighter that the colon in this
|
|
context, so that in the following example, either all or none of the
|
|
\verb\print\ statements are executed:
|
|
|
|
\begin{verbatim}
|
|
if x < y < z: print x; print y; print z
|
|
\end{verbatim}
|
|
|
|
Summarizing:
|
|
|
|
\begin{verbatim}
|
|
compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef
|
|
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
|
|
statement: stmt_list NEWLINE | compound_stmt
|
|
stmt_list: simple_stmt (";" simple_stmt)* [";"]
|
|
\end{verbatim}
|
|
|
|
Note that statements always ends in a \verb\NEWLINE\ possibly followed
|
|
by a \verb\DEDENT\.
|
|
\index{NEWLINE token}
|
|
\index{DEDENT token}
|
|
|
|
Also note that optional continuation clauses always begin with a
|
|
keyword that cannot start a statement, thus there are no ambiguities
|
|
(the `dangling \verb\else\' problem is solved in Python by requiring
|
|
nested \verb\if\ statements to be indented).
|
|
\indexii{dangling}{else}
|
|
|
|
The formatting of the grammar rules in the following sections places
|
|
each clause on a separate line for clarity.
|
|
|
|
\section{The {\tt if} statement}
|
|
\stindex{if}
|
|
|
|
The \verb\if\ statement is used for conditional execution:
|
|
|
|
\begin{verbatim}
|
|
if_stmt: "if" condition ":" suite
|
|
("elif" condition ":" suite)*
|
|
["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
It selects exactly one of the suites by evaluating the conditions one
|
|
by one until one is found to be true (see section \ref{Booleans} for
|
|
the definition of true and false); then that suite is executed (and no
|
|
other part of the \verb\if\ statement is executed or evaluated). If
|
|
all conditions are false, the suite of the \verb\else\ clause, if
|
|
present, is executed.
|
|
\kwindex{elif}
|
|
\kwindex{else}
|
|
|
|
\section{The {\tt while} statement}
|
|
\stindex{while}
|
|
\indexii{loop}{statement}
|
|
|
|
The \verb\while\ statement is used for repeated execution as long as a
|
|
condition is true:
|
|
|
|
\begin{verbatim}
|
|
while_stmt: "while" condition ":" suite
|
|
["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
This repeatedly tests the condition and, if it is true, executes the
|
|
first suite; if the condition is false (which may be the first time it
|
|
is tested) the suite of the \verb\else\ clause, if present, is
|
|
executed and the loop terminates.
|
|
\kwindex{else}
|
|
|
|
A \verb\break\ statement executed in the first suite terminates the
|
|
loop without executing the \verb\else\ clause's suite. A
|
|
\verb\continue\ statement executed in the first suited skips the rest
|
|
of the suite and goes back to testing the condition.
|
|
\stindex{break}
|
|
\stindex{continue}
|
|
|
|
\section{The {\tt for} statement}
|
|
\stindex{for}
|
|
\indexii{loop}{statement}
|
|
|
|
The \verb\for\ statement is used to iterate over the elements of a
|
|
sequence (string, tuple or list):
|
|
\obindex{sequence}
|
|
|
|
\begin{verbatim}
|
|
for_stmt: "for" target_list "in" condition_list ":" suite
|
|
["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
The condition list is evaluated once; it should yield a sequence. The
|
|
suite is then executed once for each item in the sequence, in the
|
|
order of ascending indices. Each item in turn is assigned to the
|
|
target list using the standard rules for assignments, and then the
|
|
suite is executed. When the items are exhausted (which is immediately
|
|
when the sequence is empty), the suite in the \verb\else\ clause, if
|
|
present, is executed, and the loop terminates.
|
|
\kwindex{in}
|
|
\kwindex{else}
|
|
\indexii{target}{list}
|
|
|
|
A \verb\break\ statement executed in the first suite terminates the
|
|
loop without executing the \verb\else\ clause's suite. A
|
|
\verb\continue\ statement executed in the first suited skips the rest
|
|
of the suite and continues with the next item, or with the \verb\else\
|
|
clause if there was no next item.
|
|
\stindex{break}
|
|
\stindex{continue}
|
|
|
|
The suite may assign to the variable(s) in the target list; this does
|
|
not affect the next item assigned to it.
|
|
|
|
The target list is not deleted when the loop is finished, but if the
|
|
sequence is empty, it will not have been assigned to at all by the
|
|
loop.
|
|
|
|
Hint: the built-in function \verb\range()\ returns a sequence of
|
|
integers suitable to emulate the effect of Pascal's \verb\for i := a
|
|
to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
|
|
\bifuncindex{range}
|
|
\index{Pascal}
|
|
|
|
{\bf Warning:} There is a subtlety when the sequence is being modified
|
|
by the loop (this can only occur for mutable sequences, i.e. lists).
|
|
An internal counter is used to keep track of which item is used next,
|
|
and this is incremented on each iteration. When this counter has
|
|
reached the length of the sequence the loop terminates. This means that
|
|
if the suite deletes the current (or a previous) item from the
|
|
sequence, the next item will be skipped (since it gets the index of
|
|
the current item which has already been treated). Likewise, if the
|
|
suite inserts an item in the sequence before the current item, the
|
|
current item will be treated again the next time through the loop.
|
|
This can lead to nasty bugs that can be avoided by making a temporary
|
|
copy using a slice of the whole sequence, e.g.
|
|
\index{loop!over mutable sequence}
|
|
\index{mutable sequence!loop over}
|
|
|
|
\begin{verbatim}
|
|
for x in a[:]:
|
|
if x < 0: a.remove(x)
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt try} statement}
|
|
\stindex{try}
|
|
|
|
The \verb\try\ statement specifies exception handlers and/or cleanup
|
|
code for a group of statements:
|
|
|
|
\begin{verbatim}
|
|
try_stmt: try_exc_stmt | try_fin_stmt
|
|
try_exc_stmt: "try" ":" suite
|
|
("except" [condition ["," target]] ":" suite)+
|
|
try_fin_stmt: "try" ":" suite
|
|
"finally" ":" suite
|
|
\end{verbatim}
|
|
|
|
There are two forms of \verb\try\ statement: \verb\try...except\ and
|
|
\verb\try...finally\. These forms cannot be mixed.
|
|
|
|
The \verb\try...except\ form specifies one or more exception handlers
|
|
(the \verb\except\ clauses). When no exception occurs in the
|
|
\verb\try\ clause, no exception handler is executed. When an
|
|
exception occurs in the \verb\try\ suite, a search for an exception
|
|
handler is started. This inspects the except clauses in turn until
|
|
one is found that matches the exception. A condition-less except
|
|
clause, if present, must be last; it matches any exception. For an
|
|
except clause with a condition, that condition is evaluated, and the
|
|
clause matches the exception if the resulting object is ``compatible''
|
|
with the exception. An object is compatible with an exception if it
|
|
is either the object that identifies the exception or it is a tuple
|
|
containing an item that is compatible with the exception. Note that
|
|
the object identities must match, i.e. it must be the same object, not
|
|
just an onject with the same value.
|
|
\kwindex{except}
|
|
|
|
If no except clause matches the exception, the search for an exception
|
|
handler continues in the surrounding code and on the invocation stack.
|
|
|
|
If the evaluation of a condition in the header of an except clause
|
|
raises an exception, the original search for a handler is cancelled
|
|
and a search starts for the new exception in the surrounding code and
|
|
on the call stack (it is treated as if the entire \verb\try\ statement
|
|
raised the exception).
|
|
|
|
When a matching except clause is found, the exception's parameter is
|
|
assigned to the target specified in that except clause, if present,
|
|
and the except clause's suite is executed. When the end of this suite
|
|
is reached, execution continues normally after the entire try
|
|
statement. (This means that if two nested handlers exist for the same
|
|
exception, and the exception occurs in the try clause of the inner
|
|
handler, the outer handler will not handle the exception.)
|
|
|
|
The \verb\try...finally\ form specifies a `cleanup' handler. The
|
|
\verb\try\ clause is executed. When no exception occurs, the
|
|
\verb\finally\ clause is executed. When an exception occurs in the
|
|
\verb\try\ clause, the exception is temporarily saved, the
|
|
\verb\finally\ clause is executed, and then the saved exception is
|
|
re-raised. If the \verb\finally\ clause raises another exception or
|
|
executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
|
|
the saved exception is lost.
|
|
\kwindex{finally}
|
|
|
|
When a \verb\return\ or \verb\break\ statement is executed in the
|
|
\verb\try\ suite of a \verb\try...finally\ statement, the
|
|
\verb\finally\ clause is also executed `on the way out'. A
|
|
\verb\continue\ statement is illegal in the \verb\try\ clause. (The
|
|
reason is a problem with the current implementation --- this
|
|
restriction may be lifted in the future).
|
|
\stindex{return}
|
|
\stindex{break}
|
|
\stindex{continue}
|
|
|
|
\section{Function definitions} \label{function}
|
|
\indexii{function}{definition}
|
|
|
|
A function definition defines a user-defined function object (see
|
|
section \ref{types}):
|
|
\obindex{user-defined function}
|
|
\obindex{function}
|
|
|
|
\begin{verbatim}
|
|
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
|
|
parameter_list: (parameter ",")* ("*" identifier | parameter [","])
|
|
sublist: parameter ("," parameter)* [","]
|
|
parameter: identifier | "(" sublist ")"
|
|
funcname: identifier
|
|
\end{verbatim}
|
|
|
|
A function definition is an executable statement. Its execution binds
|
|
the function name in the current local name space to a function object
|
|
(a wrapper around the executable code for the function). This
|
|
function object contains a reference to the current global name space
|
|
as the global name space to be used when the function is called.
|
|
\indexii{function}{name}
|
|
\indexii{name}{binding}
|
|
|
|
The function definition does not execute the function body; this gets
|
|
executed only when the function is called.
|
|
|
|
Function call semantics are described in section \ref{calls}. When a
|
|
user-defined function is called, the arguments (a.k.a. actual
|
|
parameters) are bound to the (formal) parameters, as follows:
|
|
\indexii{function}{call}
|
|
\indexiii{user-defined}{function}{call}
|
|
\index{parameter}
|
|
\index{argument}
|
|
\indexii{parameter}{formal}
|
|
\indexii{parameter}{actual}
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
If there are no formal parameters, there must be no arguments.
|
|
|
|
\item
|
|
If the formal parameter list does not end in a star followed by an
|
|
identifier, there must be exactly as many arguments as there are
|
|
parameters in the formal parameter list (at the top level); the
|
|
arguments are assigned to the formal parameters one by one. Note that
|
|
the presence or absence of a trailing comma at the top level in either
|
|
the formal or the actual parameter list makes no difference. The
|
|
assignment to a formal parameter is performed as if the parameter
|
|
occurs on the left hand side of an assignment statement whose right
|
|
hand side's value is that of the argument.
|
|
|
|
\item
|
|
If the formal parameter list ends in a star followed by an identifier,
|
|
preceded by zero or more comma-followed parameters, there must be at
|
|
least as many arguments as there are parameters preceding the star.
|
|
Call this number {\em N}. The first {\em N} arguments are assigned to
|
|
the corresponding formal parameters in the way descibed above. A
|
|
tuple containing the remaining arguments, if any, is then assigned to
|
|
the identifier following the star. This variable will always be a
|
|
tuple: if there are no extra arguments, its value is \verb\()\, if
|
|
there is just one extra argument, it is a singleton tuple.
|
|
\indexii{variable length}{parameter list}
|
|
|
|
\end{itemize}
|
|
|
|
Note that the `variable length parameter list' feature only works at
|
|
the top level of the parameter list; individual parameters use a model
|
|
corresponding more closely to that of ordinary assignment. While the
|
|
latter model is generally preferable, because of the greater type
|
|
safety it offers (wrong-sized tuples aren't silently mistreated),
|
|
variable length parameter lists are a sufficiently accepted practice
|
|
in most programming languages that a compromise has been worked out.
|
|
(And anyway, assignment has no equivalent for empty argument lists.)
|
|
|
|
\section{Class definitions} \label{class}
|
|
\indexii{class}{definition}
|
|
|
|
A class definition defines a class object (see section \ref{types}):
|
|
\obindex{class}
|
|
|
|
\begin{verbatim}
|
|
classdef: "class" classname [inheritance] ":" suite
|
|
inheritance: "(" [condition_list] ")"
|
|
classname: identifier
|
|
\end{verbatim}
|
|
|
|
A class definition is an executable statement. It first evaluates the
|
|
inheritance list, if present. Each item in the inheritance list
|
|
should evaluate to a class object. The class's suite is then executed
|
|
in a new execution frame (see section \ref{execframes}), using a newly
|
|
created local name space and the original global name space.
|
|
(Usually, the suite contains only function definitions.) When the
|
|
class's suite finishes execution, its execution frame is discarded but
|
|
its local name space is saved. A class object is then created using
|
|
the inheritance list for the base classes and the saved local name
|
|
space for the attribute dictionary. The class name is bound to this
|
|
class object in the original local name space.
|
|
\index{inheritance}
|
|
\indexii{class}{name}
|
|
\indexii{name}{binding}
|
|
\indexii{execution}{frame}
|
|
|
|
\chapter{Top-level components}
|
|
|
|
The Python interpreter can get its input from a number of sources:
|
|
from a script passed to it as standard input or as program argument,
|
|
typed in interactively, from a module source file, etc. This chapter
|
|
gives the syntax used in these cases.
|
|
\index{interpreter}
|
|
|
|
\section{Complete Python programs}
|
|
\index{program}
|
|
|
|
While a language specification need not prescribe how the language
|
|
interpreter is invoked, it is useful to have a notion of a complete
|
|
Python program. A complete Python program is executed in a minimally
|
|
initialized environment: all built-in and standard modules are
|
|
available, but none have been initialized, except for \verb\sys\
|
|
(various system services), \verb\builtin\ (built-in functions,
|
|
exceptions and \verb\None\) and \verb\__main__\. The latter is used
|
|
to provide the local and global name space for execution of the
|
|
complete program.
|
|
\bimodindex{sys}
|
|
\bimodindex{__main__}
|
|
\bimodindex{builtin}
|
|
|
|
The syntax for a complete Python program is that for file input,
|
|
described in the next section.
|
|
|
|
The interpreter may also be invoked in interactive mode; in this case,
|
|
it does not read and execute a complete program but reads and executes
|
|
one statement (possibly compound) at a time. The initial environment
|
|
is identical to that of a complete program; each statement is executed
|
|
in the name space of \verb\__main__\.
|
|
\index{interactive mode}
|
|
|
|
Under {\UNIX}, a complete program can be passed to the interpreter in
|
|
three forms: with the {\bf -c} {\it string} command line option, as a
|
|
file passed as the first command line argument, or as standard input.
|
|
If the file or standard input is a tty device, the interpreter enters
|
|
interactive mode; otherwise, it executes the file as a complete
|
|
program.
|
|
\index{UNIX}
|
|
\index{command line}
|
|
\index{standard input}
|
|
|
|
\section{File input}
|
|
|
|
All input read from non-interactive files has the same form:
|
|
|
|
\begin{verbatim}
|
|
file_input: (NEWLINE | statement)*
|
|
\end{verbatim}
|
|
|
|
This syntax is used in the following situations:
|
|
|
|
\begin{itemize}
|
|
|
|
\item when parsing a complete Python program (from a file or from a string);
|
|
|
|
\item when parsing a module;
|
|
|
|
\item when parsing a string passed to \verb\exec()\;
|
|
\bifuncindex{exec}
|
|
|
|
\item when parsing a file passed to \verb\execfile()\;
|
|
\bifuncindex{execfile}
|
|
|
|
\end{itemize}
|
|
|
|
\section{Interactive input}
|
|
|
|
Input in interactive mode is parsed using the following grammar:
|
|
|
|
\begin{verbatim}
|
|
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
|
|
\end{verbatim}
|
|
|
|
Note that a (top-level) compound statement must be followed by a blank
|
|
line in interactive mode; this is needed to help the parser detect the
|
|
end of the input.
|
|
|
|
\section{Expression input}
|
|
\index{input}
|
|
|
|
There are two forms of expression input. Both ignore leading
|
|
whitespace.
|
|
|
|
The string argument to \verb\eval()\ must have the following form:
|
|
\bifuncindex{eval}
|
|
|
|
\begin{verbatim}
|
|
eval_input: condition_list NEWLINE*
|
|
\end{verbatim}
|
|
|
|
The input line read by \verb\input()\ must have the following form:
|
|
\bifuncindex{input}
|
|
|
|
\begin{verbatim}
|
|
input_input: condition_list NEWLINE
|
|
\end{verbatim}
|
|
|
|
Note: to read `raw' input line without interpretation, you can use the
|
|
built-in function \verb\raw_input()\ or the \verb\readline()\ method
|
|
of file objects.
|
|
\obindex{file}
|
|
\index{input!raw}
|
|
\index{raw input}
|
|
\bifuncindex{raw_index}
|
|
\ttindex{readline}
|
|
|
|
\input{ref.ind} % The index
|
|
|
|
\end{document}
|